Liminal
Etymology
Directly from Latin limen, "threshold." A doorway is neither quite inside nor outside; you occupy it only while crossing. Anthropologists took that physical image and stretched it to psychological, social, and cultural thresholds, and the "in-between" sense followed naturally.
adjective · /LIM-ih-nul/
Existing in an in-between state: no longer one thing, not yet the next.
What it names
The strange, suspended quality of transitions — adolescence, immigration, the months after graduating, an airport at 3 a.m.
Example
There’s a liminal week between quitting one job and starting the next when you belong to neither.
Related idea
Threshold is the literal root; interstitial is the more clinical word for the space between things.