Both Uses

phase
in
A Man Named Dave
(Edited)

For almost a year as the SR-71 was being phased out, I had signed paperwork swearing to absolute secrecy about my involvement with the Stealth program, even though the aircraft had already been revealed to the public.
p. 239.2 *

phased = gradually eliminated
By mid-January 1991, as the air force generals briefed us on the probable losses during the initial phase of the air campaign, the possibility of losing every third person opened my eyes.
p. 247.1 *

phase = stage (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.