Many times we see this word. Sometimes I've seen it used correctly sometimes incorrectly, in the association of devotees. So I thought I would put in my 2 cents about this and then add some quotes by Srila Prabhupada about it. Mayavadi essentially means one who thinks that Krsna's transcendental form, name, pastimes, birth etc are material. This is an offense. It doesn't mean only that one is an impersonalist. For example the 4 Kumaras were impersonalists originally until they came in to contact with Krnsa and then they became devotees. They were never mayavadis. But when one hears about the transcendental nature of Krsna's pastimes and then continues to think they are material that is mayavadi. And the worst are those who take a very active stance in opposition to the transcendental nature of Krsna's pastimes, body etc. They cannot tolerate the concept that Krsna as the Supreme Personality of Godhead can appear in His transcendental form in the material world. They actively offend Krsna with every word they say about Him, and they also offend His pure devotee. Such people's association is to be avoided like one would avoid drinking poison.

Here are some quotes:

TRANSLATION

I have studied the path of understanding self-knowledge from my spiritual master, the Personality of Godhead, and thus after circumambulating Him I have come to this place, very much aggrieved due to separation.

PURPORT

Sri Uddhava's actual life is the direct symbol of the catuh-sloki Bhagavatam enunciated first to Brahmaji by the Personality of Godhead. These four very great and important verses from Srimad-Bhagavatam are particularly taken out by the Mayavadi speculators, who construe a different purport to suit their impersonal view of monism. Here is the proper answer to such unauthorized speculators. The verses of Srimad-Bhagavatam are purely theistic science understandable by the postgraduate students of Bhagavad-gita, The unauthorized dry speculators are offenders at the lotus feet of the Lord Sri Krsna because they distort the purports of Bhagavad-gita and Srimad-Bhagavatam to mislead the public and prepare a direct path to the hell known as Andha-tamisra. As confirmed in Bhagavad-gita (16.20) such envious speculators are without knowledge and are surely condemned life after life. They unnecessarily take shelter of Sripada Sankaracarya, but he was not so drastic as to commit an offense at the lotus feet of Lord Krsna. According to Lord Sri Caitanya Mahaprabhu, Sripada Sankaracarya preached the Mayavada philosophy for a particular purpose. Such a philosophy was necessary to defeat the Buddhist philosophy of the nonexistence of the spirit soul, but it was never meant for perpetual acceptance. It was an emergency. Thus Lord Krsna was accepted by Sankaracarya as the Supreme Personality of Godhead in his commentation on Bhagavad-gita. Since he was a great devotee of Lord Krsna, he did not dare write any commentary on Srimad-Bhagavatam because that would have been a direct offense at the lotus feet of the Lord. But later speculators, in the name of Mayavada philosophy, unnecessarily make their commentary on the catuh-sloki Bhagavatam without any bona fide intent.
The monistic dry speculators have no business in the Srimad-Bhagavatam because this particular Vedic literature is forbidden for them by the great author himself. Srila Vyasadeva has definitely forbidden persons engaged in religiosity, economic development, sense gratification and, finally, salvation, from trying to understand Srimad-Bhagavatam, which is not meant for them (Bhag. 1.1.2). Sripada Sridhara Svami, the great commentator on Srimad-Bhagavatam, has definitely forbidden the salvationists or monists to deal in Srimad-Bhagavatam. It is not for them. Yet such unauthorized persons perversely try to understand Srimad-Bhagavatam, and thus they commit offenses at the feet of the Lord, which even Sripada Sankaracarya dared not do. Thus they prepare for their continuation of miserable life. It should be particularly noted herein that Uddhava studied the catuh-sloki Bhagavatam directly from the Lord, who spoke them first to Brahmaji, and this time the Lord explained more confidentially the self-knowledge mentioned as the paramam sthitim. Upon learning such self-knowledge of love, Uddhava felt very much aggrieved by feelings of separation from the Lord. Unless one is awakened to the stage of Uddhava--everlastingly feeling the separation of the Lord in transcendental love, as exhibited by Lord Caitanya also--one cannot understand the real import of the four essential verses of Srimad-Bhagavatam. One should not indulge in the unauthorized act of twisting the meaning and thereby putting himself on the dangerous path of offense.
(Srimad Bhagavatam 3.4.20)

The dangerous Mayavada theory set forth by Sankaracarya--that God is impersonal--does not tally with the injunctions of the Vedas. Sri Caitanya Mahaprabhu therefore described the Mayavadi philosophers as the greatest offenders against the Personality of Godhead. According to the Vedic system, one who does not abide by the orders of the Vedas is called a nastika, or atheist. When Lord Buddha preached his theory of nonviolence, he was obliged to deny the authority of the Vedas, and for this reason he was considered by the followers of the Vedas to be a nastika. But although Sri Caitanya Mahaprabhu very clearly enunciated that the followers of Lord Buddha's philosophy are nastikas, or atheists, because of their denial of the authority of the Vedas, He considered the Sankarites, who wanted to establish Vedic authority by trickery and who actually followed the Mayavada philosophy of Buddha's school, to be more dangerous than the Buddhists themselves. The Sankarite philosophers' theory that we have to imagine a shape of God is more dangerous than denial of the existence of God. Notwithstanding all the philosophical theorizing by atheists or Mayavadis, the followers of Krsna consciousness rigidly live according to the injunctions given in Bhagavad-gita, which is accepted as the essence of all Vedic scripture. In Bhagavad-gita (18.46) it is said:

yatah pravrttir bhutanam
yena sarvam idam tatam
sva-karmana tam abhyarcya
siddhim vindati manavah

"By worship of the Lord, who is the source of all beings and who is all-pervading, man can, in the performance of his own duty, attain perfection." This indicates that the Supreme Personality of Godhead is the original source of everything, as described in the Vedanta-sutra (janmady asya yatah). The Lord Himself also confirms in Bhagavad-gita, aham sarvasya prabhavah: "I am the origin of everything." The Supreme Personality of Godhead is the original source of all emanations, and at the same time, as Paramatma, He is spread all over existence. The Absolute Truth is therefore the Supreme Personality of Godhead, and every living being is meant to satisfy the Supreme Godhead by performing his respective duty (sva-karmana tam abhyarcya). Maharaja Prthu wanted to introduce this formula amongst his citizens.
The most important point in human civilization is that while one engages in different occupational duties, he must try to satisfy the Supreme Lord by the execution of such duties. That is the highest perfection of life. Svanusthitasya dharmasya samsiddhir hari-tosanam: by discharging one's prescribed duty, one can become very successful in life if he simply satisfies the Supreme Personality of Godhead. The vivid example is Arjuna. He was a ksatriya, his duty was to fight, and by executing his prescribed duty he satisfied the Supreme Lord and therefore became perfect. Everyone should follow this principle. The atheists, who do not, are condemned in Bhagavad-gita (16.19) by the following statement: tan aham dvisatah kruran samsaresu naradhaman. In this verse it is clearly said that persons who are envious of the Supreme Personality of Godhead are the lowest of mankind and are very mischievous. Under the regulative principles of the Supreme, such mischievous persons are thrown into the darkest region of material existence and are born of asuras, or atheists. Birth after birth, such asuras go still further down, finally to animal forms like those of tigers or similar ferocious beasts. Thus for millions of years they have to remain in darkness without knowledge of Krsna.
The Supreme personality of Godhead is known as Purusottama, or the best of all living entities. He is a person like all other living entities, but He is the leader or the best of all living beings. That is stated in the Vedas also. Nityo nityanam cetanas cetananam. He is the chief of all eternals, the chief of all living entities, and He is complete and full. He has no need to derive benefit by interfering with the affairs of other living entities, but because He is the maintainer of all, He has the right to bring them to the proper standard so that all living entities may become happy. A father wants all of his children to become happy under his direction. Similarly, God, or Krsna, the Supreme Personality of Godhead, has the right to see that all living entities are happy. There is no possibility of becoming happy within this material world. The father and the sons are eternal, but if a living entity does not come to the platform of his eternal life of bliss and knowledge, there is no question of happiness. Although Purusottama, the best of all living entities, has no benefit to derive from the common living entities, He does have the right to discriminate between their right and wrong ways. The right way is the path of activities meant to satisfy the Supreme Personality of Godhead, as we have already discussed (svanusthitasya dharmasya samsiddhir hari-tosanam). A living entity may engage in any occupational duty, but if he wants to have perfection in his duties, he must satisfy the Supreme Lord. As such, one who pleases Him gets better facilities for living, but one who displeases Him gets involved in undesirable situations.
(Srimad Bhagavatam 4.21.27)