Involuntary celibacy is the absence in human sexuality of intimate relationships or sexual intercourse for reasons other than voluntary celibacy, asexuality, antisexualism, or sexual abstinence. The term (which is sometimes shortened to incel) describes those who, despite being open to sexual intimacy and potential romance with someone and also making active, repeated efforts towards such an end, cannot cause any such end(s) to occur with any significant degree of regularity—or even at all.

As a concept, involuntary celibacy distinguishes itself from other various celibacy types by two major overall characteristics: First, it is explicitly not dependent on the choices or actions of the individual, and is instead a pattern-like, semi-perpetual, essentially unchanging situation, even if the person tries in good faith to change it by improving their looks and social skills to try to attract sexual partners. Second, involuntarily celibate individuals are at a complete or near-complete lack for intimate physical connection for very long spans of time—several years and even sometimes decades at a time, not merely weeks or months—while also being at a complete or near-complete lack of opportunities for sexual advancement in the first place, thereby making betterment of their own sexuality through accumulation of "sexual experience" impossible.

What makes involuntary celibacy an especially difficult condition for its sufferers to deal with is the fact that most of the time the circumstance cannot be explained through external personal factors—most incels, based on inquests by researchers into the population, are not especially physically unattractive, and most resemble in an interpersonal sense their peers who are not involuntarily celibate. Although a few of the involuntarily celibate population may have discernible personality disorders that preclude current and future sexual opportunities, the small amount of research done on this subject indicates that the incel population are on the whole socially normal, healthy individuals whose frustration is merely a product of their lack of sex, and not vice versa. This makes an individual's involuntarily celibate situation extremely difficult to resolve through the standard psychological methods of pinpointing and "fixing" internal and external life circumstances.

There are critics who claim that involuntary celibacy not only does not, but also that it cannot, exist in the first place, because to these critics the 'actual' cause is such people's lack of taking responsibility for their own sexual prowess and lack of initiative at knowing how to improve it. Such critics allege that self-proclaimed incels are simply affixing themselves with a label so the label can be used in self-defense. Incels themselves continue to insist that they are not "making excuses"; that involuntary celibacy is very real and can be very emotionally destructive; and some, on the assumption that incel is a valid sexological condition, are actively fighting to get incel officially recognised in the sexual science field.