The insertion of the male's erect penis into the anus and rectum of a male or female partner.
Although anal penetration is more commonly called "anal sex," "anal intercourse," or "anal sexual intercourse," we avoid those terms because they imply a correspondence between penile-vaginal sex, which is true genital-genital intercourse, and anal penetration, which is the fetishistic penetration of a non-genital organ of excretion.
Among gay-identified men, this painful act of penetration of one man by another is far more about power, brutality, and domination, than it is about love or even sex.
As one Alliance member says, "There's no male connection, no bond, just raw lust. It's not sex or love, it's violence and power. The problem is implied in the culturally-coerced masculine/feminine roles."
Those "culturally-coerced masculine/feminine roles" in anal penetration, which are most commonly known as top (for anally-insertive) and bottom (for anally-receptive), create severe power inequalities and a hieratic structure among men who practice anal penetration.
See, in that regard, and to better understand the power relationships between insertive and receptive actors, entries on Dom/Sub, effeminacy, intergendered, top, and bottom.
In addition, anal penetration is dirty -- because fecal material is always present; dangerous -- because it's a vector for virtually every STI and frequently results in mechanical damage to the rectum and/or anus; and degrading -- because it turns a man into a proxy for a woman, and a woman into a mere receptacle for a man's penis and ejaculate.
Despite these many problems, including the loss of more than half-a-million gay and bi male lives to an anally-transmitted disease, and despite the fact that prior to 1975 anal penetration was not the pre-eminent gay male sex act, for almost three decades anal penetration has been unceasingly exalted and romanticized by mainstream gay male culture, and so remains the defining act of that culture, a dominant culture of anal penetration.
For more about the history of anal penetration among gay men and the cultural forces which currently support anal penetration, see our Man2Man Alliance policy paper Multipartnered Pansexualism or Heroic Love.
Readers who wish to better understand the role of anal penetration in the concurrent epidemics of sexually transmitted disease which afflict the gay male community may refer to our Man2Man Alliance policy paper, an anus is not a vagina, Part 1: The Biology of Anal Penetration.