The capacity to FEEL a relationship's safety when you're not in it. Often impaired in BPD — when you're not present, your love stops feeling real, even though they intellectually know it's still there.
You've told her you love her every day for two years. The text gap on a busy day still produces real terror — "you're done with me" — not as a guess but as a felt fact. Object constancy lets people remember love exists; emotion permanence lets them feel it. Both can be impaired.