A man is an adult male human. Before adulthood, a male child or adolescent is referred to as a boy.

Like most other male mammals, a man's genome usually inherits an X chromosome from the mother and a Y chromosome from the father. Sex differentiation of the male fetus is governed by the SRY gene on the Y chromosome. During puberty, hormones which stimulate androgen production result in the development of secondary sexual characteristics that result in even more differences between the sexes. These include greater muscle mass, greater height, the growth of facial hair and a lower body fat composition. Male anatomy is distinguished from female anatomy by the male reproductive system, which includes the testicles, sperm ducts, prostate gland and epididymides, and penis. Secondary sex characteristics include a narrower pelvis and hips, and smaller breasts and nipples.

Throughout human history, traditional gender roles have often defined men's activities and opportunities. Men often face conscription into military service or are directed into professions with high mortality rates. Many religious doctrines stipulate certain rules for men, such as religious circumcision. Men are over-represented as both perpetrators and victims of violence.

Trans men have a gender identity that does not align with their female sex assignment at birth, while intersex men may have sex characteristics that do not fit typical notions of male biology.

The English term "man" is derived from the Proto-Indo-European root *man- (see Sanskrit/Avestan manu-, Slavic mǫž "man, male"). More directly, the word derives from Old English mann. The Old English form primarily meant "person" or "human being" and referred to men, women, and children alike. The Old English word for "man" as distinct from "wif"/"woman" or "child" was wer. Mann only came to mean "man" in Middle English, replacing wer, which survives today only in the compounds "werewolf" (from Old English werwulf, literally "man-wolf"), and "wergild", literally "man-payment".

In humans, sperm cells carry either an X or a Y sex chromosome. If a sperm cell carrying a Y chromosome fertilizes the female ovum, the offspring will have a male karyotype (XY). The SRY gene is typically found on the Y chromosome and causes the development of the testes, which in turn govern other aspects of male sex differentiation. Sex differentiation in males proceeds in a testes-dependent way while female differentiation is not gonad dependent.

Primary sex characteristics (or sex organs) are characteristics that are present at birth and are integral to the reproductive process. For men, primary sex characteristics include the penis and testicles.

Adult humans exhibit sexual dimorphism in many other characteristics, many of which have no direct link to reproductive ability. Humans are sexually dimorphic in body size, body structure, and body composition. Men tend to be taller and heavier than women, and adjusted for height, men tend to have greater lean and bone mass than women, and lower fat mass.