Paraphilic infantilism, also known as adult baby (or "AB", for short), is a form of ageplay that involves role-playing a regression to an infant-like state. Like other forms of adult play, depending on the context and desires of the people involved, paraphilic infantilism may be expressed as a non-sexual fetish, kink, or simply as a comforting platonic activity. People who practice adult baby play are often colloquially referred to (by themselves and others) as "adult babies", or "ABs".

Behaviors vary, but may include things such as wearing and using diapers, drinking from a baby bottle or sucking on a pacifier, and (when done with others) engaging in gentle, nostalgic and nurturing experiences, baby talk, or BDSM power dynamics involving masochism, coercion, punishment or humiliation.

Paraphilic infantilism is often associated with diaper fetishism, a separate but related activity in which people derive pleasure or ecstasy from themselves or others wearing or using diapers, but without necessarily involving any form of ageplay. People with a diaper fetish are often informally called "diaper lovers", or "DLs". In practice, however, these strict labels do not always reflect the true diversity of expression. As such, when considered together, paraphilic infantilism and diaper fetishism form a spectrum of behaviors that are often colloquially referred to under the umbrella term "adult baby/diaper lover", or "AB/DL" (also written "ABDL").

Like other sexual fetishes (paraphilias), there is no single recognized psychological origin for paraphilic infantilism and very little research has been done on the subject as of yet. A variety of theories have been proposed for fetish development in general, including unique lovemaps, imprinting or altered erotic targets, though no scientific consensus has emerged. Though it varies from person to person, paraphilic infantilism may sometimes be linked to masochism, urolagnia, garment fetishes, coprophilia, or other consensual kinks and fetishes.

Paraphilic infantilists (usually colloquially called "adult babies" or "ABDLs") are people who derive sexual pleasure and/or emotional comfort from imagining themselves as, and pretending to be, children. It is a specific form of ageplay and, more broadly, sexual role-play, that can be practiced alone or with consenting adult partners. As a paraphilia (a fetish or kink), paraphilic infantilism may represent an atypical sexual interest but is not usually associated with any psychological disorder or poor mental health, and adult baby play can be considered a conscious and valid act of sexual expression. In addition, some people participate in adult baby play purely for the positive feeling and emotional comfort associated with childhood and being cared for; to what degree (if at all) eroticism plays a role depends heavily on the participants and context.

Whether performed solo or with others, one large part of paraphilic infantilism can be dressing in childlike clothing (diapers, onesies, plastic pants, bibs, etc.) and using related accessories and items (pacifiers, nursery-style furniture, etc.) along with other behaviors, such as eating while sitting in a highchair, sleeping in a crib, drinking from a baby bottle or sippy cup, or eating off of children's tableware. As such, depending on the person, it may or may not be associated with a variety of garment and object fetishes, like diaper fetishism.

Another large part of adult baby expression can be behaving like an infant or toddler and engaging in activities such as crawling, sitting on the floor and playing with toys or games, coloring in children's coloring books and activity books, consuming food or drinks associated with childhood (juiceboxes, chicken nuggets, gummy candies, etc.), breastfeeding, using ("wetting" or "soiling") diapers or other clothes and being changed, taking naps, engaging in baby talk, etc.

When engaging in paraphilic infantilism with partners, various dynamics are possible and participants may take up a variety of different roles, including one or more participants acting as "littles" (adult age players), while others may function as "caregivers" (or "CGs") or "switches" (people whose role may change during or between scenes). This relatively common dynamic is generally referred to as "caregiver/little", or "CG/L".