an individual who is currently legally married to a child's natural or adoptive parent but has no biological or adoptive parental relationship to the child.

the spouse of your parent by a subsequent marriage

a live-in or visited adult who volunteers part-time or full-time guidance, protection, nurturance, transportation, (often), shelter, and maybe financial support, for the minor or grown biokid/s of their primary partner

an adult who (a) chooses a single parent as their primary partner, and (b) wants to - and/or is expected to - accept responsibility for co-nurturing their partner's child/ren (stepchildren) from a prior relationship