Meaning of “passing” - Learner’s Dictionary

passing

noun uk us /ˈpɑːsɪŋ/
the passing of time/years

the way that time passes:

With the passing of time their love had changed.
in passing

If you say something in passing, you talk about one thing briefly while talking mainly about something else:

She mentioned in passing that she'd seen Stuart.

(Definition of “passing noun” from the Cambridge Learner’s Dictionary © Cambridge University Press)