In the field of zoology, an alloparent is any individual other than the mother and father who help take responsibility for the young. It is broadly a synonym for 'step-parent' or 'caretaker', although it does not imply that the biological parents are dead or otherwise out of the picture. It is common for older siblings and grandparents to act as alloparents.
Alloparental relationships may be subdivided into 'true alloparents', indicating voluntary care through group parenting, extended families, adoption, and the like; and 'misdirected alloparents', in which case the alloparent does not know that they have been tricked into parenting another's young, usually through cuckoldry or brood parasitism. Unless otherwise specified, alloparental relationships are voluntary and provide some form of benefit to the alloparent, even if this is simply some form of kin altruism.
This sort of behavior is mostly seen in social species, and unsurprisingly this term is often used in primatology and physical anthropology, although it is rarely used in cultural anthropology. The term does, surprisingly, pop up in sociology from time to time.