Both Uses

phase
in
The Catcher in the Rye
(Edited)

I'm just going through a phase right now.
p. 20.9 *

phase = a period of time when someone exhibits unusual behavior
Everybody goes through phases and all, don't they?
p. 21.1

phases = periods of time when someone exhibits unusual behavior

Definitions:

(1) (phase as in: a critical phase)
a period of time or a stage during the development of something
(2) (phase as in: she's going through a phase)
a period of time when someone exhibits unusual behavior
(3) (phase as in: phased out over three years)
make a change gradually (phase in = gradually add; phase out = gradually removed)
(4) (meaning too rare to warrant focus)
More specialized and less common senses of the word are found in astronomy, physics, chemistry, and zoology. Consult a comprehensive dictionary if you wish to see those. All senses have to do with stages or periods of time.