stepparent

noun

1. One's parent's spouse who is not one's biological parent.

   “The age at which a stepparent entered a child’s life, and whether they lived together and for how long, influences the quality of the relationship, studies show. […] As parents age, “there’s a lot of negotiation and uncertainties,” Dr. Silverstein said. “Who has the right to make decisions for stepparents becomes murky.””