A microaggression is a term used for commonplace verbal, behavioral or environmental slight, whether intentional or unintentional, that communicates hostile, derogatory, or negative attitudes toward members of marginalized groups. The term was coined by Harvard University psychiatrist Chester M. Pierce in 1970 to describe insults and dismissals which he regularly witnessed non-black Americans inflicting on African Americans. By the early 21st century, use of the term was applied to the casual disparagement of any socially marginalized group, including LGBT, poor, and disabled people. Psychologist Derald Wing Sue defines microaggressions as "brief, everyday exchanges that send denigrating messages to certain individuals because of their group membership". In contrast to aggression, in which there is usually an intent to cause harm, persons making microagressive comments may be otherwise well-intentioned and unaware of the potential impact of their words.

A number of scholars and social commentators have criticized the concept of microaggression for its lack of a scientific basis, over-reliance on subjective evidence, and promotion of psychological fragility. Critics argue that avoiding behaviors that one interprets as microaggressions restricts one's own freedom and causes emotional self-harm, and that employing authority figures to address microaggressions (i.e. call-out culture) can lead to an atrophy of those skills needed to mediate one's own disputes. Some argue that, because the term "microaggression" uses language connoting violence to describe verbal conduct, it can be abused to exaggerate harm, resulting in retribution and the elevation of victimhood.

D. W. Sue, who popularized the term microaggressions, has expressed doubts on how the concept is being used: "I was concerned that people who use these examples would take them out of context and use them as a punitive rather than an exemplary way." In the 2020 edition of his book with Lisa Spanierman and in a 2021 book with his doctoral students, Dr. Sue introduces the idea of "microinterventions" as potential solutions to acts of microaggression.

Microaggressions are common, everyday slights and comments that relate to various aspects of one's appearance or identity such as class, gender, sex, sexual orientation, race, ethnicity, mother tongue, age, body shape, disability, or religion. They are thought to spring from unconsciously held prejudices and beliefs which may be demonstrated consciously or unconsciously through daily interactions. Although these communications typically appear harmless to observers, they are considered a form of subtle or everyday discrimination, like covert racism. Microaggressions differ from what Pierce referred to as "macroaggressions" in the context of racial discrimination: these are more extreme forms of racism (such as lynchings or beatings), differentiated due to their ambiguity, size and commonality. Today, researchers do not use the prefix "micro-" to mean the aggressions have a small impact, but instead to emphasize how subtly they take place, which can make them hard to point out. In fact, research on the impacts of repeated microaggressions towards members of minority groups suggests that microaggressions negatively affect mental health and are related to minority stress.

Microaggressions are experienced by most stigmatized individuals and occur on a regular basis. These can be particularly stressful for people on the receiving end as the harm or bias inherent to a microaggression is easily denied by those committing them. Because microaggressions are rooted in bias, which is societally taught, they can be committed by members of dominant social groups or members of marginalized groups. The impact of microaggressions is shaped by the dilemma of unconscious bias: perpetrators of microaggressions may view their actions or motivations differently than targets do, and perspectives diverge more when it is easy for offenders to ignore or dismiss implicit bias if it is in fact present. Additionally, this bias is most invisible to members of dominant social groups, and it is easy for observers to dismiss a microaggression as trivial and harmless. This makes it harder for a target to respond to a microaggression without backlash. The key factor here is that microaggressions are harder to detect by members of the dominant culture, as they are often unaware they are causing harm.

Microaggressions are often divided into three categories: each can be verbal or action-based, but they are united by their effect of insulting someone due to their membership in a marginalized group. All categories may occur between individuals or through an environment, via social norms or policies. Categories of microaggressions are usually called microinsults, for demeaning, rude, stereotyping or insensitive behavior; microinvalidations, for behavior that excludes or denies someone else's experiences; and microassaults, for more explicit discriminatory actions or insults. Microassaults may be an exception to the generally unconscious and subtle nature of microaggressions: some of the behaviors researchers have classified as microassault are so traumatic and overt, and perhaps frequently rooted in conscious bias, that some researchers have proposed reclassifying them outside of microaggressions.

Microaggressions were originally studied in the context of racial discrimination in the U.S., but researchers later studied their impact on many historically and presently marginalized social groups. The taxonomies and themes documented for microaggressions are often based on the identity type that a microaggression targets. Microaggressions can also be intersectional, targeting people based on more than one aspect of their identity.

Social scientists Sue, Bucceri, Lin, Nadal, and Torino (2007) described microaggressions as "the new face of racism", saying that the nature of racism has shifted over time from overt expressions of racial hatred and hate crimes, toward expressions of aversive racism, such as microaggressions, that are more subtle, ambiguous, and often unintentional. Sue says this has led some Americans to believe wrongly that non-white Americans no longer suffer from racism. One example of such subtle expressions of racism is Asian students being either pathologized or penalized as too passive or quiet. An incident that caused controversy at UCLA occurred when a teacher corrected a student's use of "indigenous" in a paper by changing it from upper- to lowercase.