Both Uses

phase
in
The Heir, by Kiera Cass
(Edited)

Twenty years ago, Dad's first act as king was to dissolve the castes, and the old system had been phased out slowly over my lifetime.
p. 2.4 *

phased = gradually eliminated
I hadn't thought about that, how he'd seen me go through the many phases of growing up, both on and off screen.
p. 109.7 *

phases = stages (of the development of something)

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.