[ noun ] a person who acts as parent and guardian for a child in place of the child's natural parents but without legally adopting the child

Used in print

(Robert Heinlein, Stranger in a Strange Land....)

Jubal said that his night-sight probably came from the conditions in which he had grown_up , and Mike grokked this was true but grokked that there was more to_it ; his foster_parents had taught him to see .