An allomother is a female who shows maternal behavior to an offspring who is not her own. This usually refers to a social animal, including primates (e.g., humans), but may also refer to an animal that is not particularly social, but has been tricked into caring for another's child, (e.g., a dunnock tricked into raising a cuckoo's egg).
The word literally means 'other mother', from the root 'allo-' + 'mother'.
Related terms include allomaternal, allopaternal, and alloparent.