Ad hominem (Latin for 'to the person'), short for argumentum ad hominem, refers to several types of arguments where the speaker attacks the character, motive, or some other attribute of the person making an argument rather than the substance of the argument itself. This avoids genuine debate by creating a diversion often using a totally irrelevant, but often highly charged attribute of the opponent's character or background. The most common form of this fallacy is "A" makes a claim of "fact", to which "B" asserts that "A" has a personal trait, quality or physical attribute that is repugnant thereby going off-topic, and hence "B" concludes that "A" has their "fact" wrong – without ever addressing the point of the debate.

Other uses of the term ad hominem are more traditional, referring to arguments tailored to fit a particular audience, and may be encountered in specialized philosophical usage. These typically refer to the dialectical strategy of using the target's own beliefs and arguments against them, while not agreeing with the validity of those beliefs and arguments. Ad hominem arguments were first studied in ancient Greece; John Locke revived the examination of ad hominem arguments in the 17th century.

A common misconception is that an ad hominem attack is synonymous with an insult. This is not true, although some ad hominem arguments may be considered insulting by the recipient.

The various types of ad hominem arguments have been known in the West since at least the ancient Greeks. Aristotle, in his work Sophistical Refutations, detailed the fallaciousness of putting the questioner but not the argument under scrutiny. His description was somewhat different from the modern understanding, referring to a class of sophistry that applies an ambiguously worded question about people to a specific person. The proper refutation, he wrote, is not to debate the attributes of the person (solutio ad hominem) but to address the original ambiguity. Many examples of ancient non-fallacious ad hominem arguments are preserved in the works of the Pyrrhonist philosopher Sextus Empiricus. In these arguments, the concepts and assumptions of the opponents are used as part of a dialectical strategy against them to demonstrate the unsoundness of their own arguments and assumptions. In this way, the arguments are to the person (ad hominem), but without attacking the properties of the individuals making the arguments. This kind of argument is also known as "argument from commitment".

Italian Galileo Galilei and British philosopher John Locke also examined the argument from commitment, a form of the ad hominem argument, meaning examining an argument on the basis of whether it stands true to the principles of the person carrying the argument. In the mid-19th century, the modern understanding of the term ad hominem started to take shape, with the broad definition given by English logician Richard Whately. According to Whately, ad hominem arguments were "addressed to the peculiar circumstances, character, avowed opinions, or past conduct of the individual".

Over time, the term acquired a different meaning; by the beginning of the 20th century, it was linked to a logical fallacy, in which a debater, instead of disproving an argument, attacked their opponent. This approach was also popularized in philosophical textbooks of the mid-20th century, and it was challenged by Australian philosopher Charles Leonard Hamblin in the second half of the 20th century. In a detailed work, he suggested that the inclusion of a statement against a person in an argument does not necessarily make it a fallacious argument since that particular phrase is not a premise that leads to a conclusion. While Hamblin's criticism was not widely accepted, Canadian philosopher Douglas N. Walton examined the fallaciousness of the ad hominem argument further. Except in specialized philosophical discourse, contemporary use of the term ad hominem describes a direct attack on the character and ethos of a person, in an attempt to refute their argument.

The Latin phrase argumentum ad hominem stands for 'argument against the person'. In this context, ad means 'against', but could also mean 'to' or 'towards'.

The terms ad mulierem and ad feminam have been used specifically when the person receiving the criticism is female but the term hominem (accusative of homo) is gender-neutral in Latin, and merely implies that the recipient of the insult is an early hominid, rather than a culturally established modern human.