Mommy/boy (also known as Mommy/lil boy or referred to as Mommy's boy fetish) is a type of adult relationship found both in BDSM and vanilla couples. The female partner in such a relationship takes the role of a "mommy" figure: authoritative, caring, gentle, strict. The male partner takes a role that is inspired by that of a "little boy" or son: permitted to be somewhat childish, tending to emotional outbursts such as joy, excitement or anger, sometimes good and obedient and sometimes naughty or bratty.
The "mommy" will typically, according to her role, make decisions, define rules and set limits of what is permitted for the "boy" and what isn't. The "boy" puts himself under her guidance, but has the security of knowing he is accepted, loved and protected, and the freedom to let out his inner child without embarrassment.
The partners normally do not pretend to really be mother and son, and the entire thing is not the same as a big roleplay with ageplay. A roleplay could hardly be kept up 24/7. It is more like a "flavor" given to a normal adult relationship, and the partners ideally don't feel as if they are acting. The "mommy" has both a parental role as well as being a sex partner. Sexual intimacy between the partners is okay and doesn't have the feeling of incest or child abuse for most partners, because after all they are still two adults with sexual needs. This is often called Lifestyle Ageplay.
In BDSM or BDSM-leaning couples, "mommy's boy" can expect to get punished for offenses he commits. This will typically not be punishments of the kind you find in Mistress/slave relationships, but more like those you find in the traditional punishment of children: scolding and spanking being most popular. Other punishments the partners in this kind of relationship may like include corner time, mouth washing or diaper discipline.
Such punishment is consensual, even when there may be acting of non-consensuality as part of a roleplay (See: consensual non-consent). The "mommy" will know and respect the "boy's" disciplinary preferences and limits. As the "mommy" takes a role that overlaps with that of a BDSM dominant, she is also known as Mommy Domme.