Both Uses

phase
in
The Best of Me
(Edited)

At first, they clung to the hope that Amanda was simply going through a rebellious phase, and they tried to ignore it.
p. 18.8 *

phase = a stage in the development of something
I saw it when she first came to see me, and I guess I hoped it was a phase, but the more she came to visit, the worse she seemed to be feeling.
p. 218.4 *

phase = a period 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.