All 5 Uses

phase
in
Never Let Me Go
(Edited)

This was all a long time ago so I might have some of it wrong; but my memory of it is that my approaching Tommy that afternoon was part of a phase I was going through around that time—something to do with compulsively setting myself challenges—and I'd more or less forgotten all about it when Tommy stopped me a few days later.
p. 13.1 *

phase = a period of time when someone exhibits unusual behavior
The boys were going through a phase of "enjoying" rounders in order to flirt with us, so there were over thirty of us that afternoon.
p. 79.3 *

phase = a period of time or a stage during the development of something
It must have been just before my talk with him by the pond; a time, I suppose, when Tommy was still coming out of that phase of being teased and taunted.
p. 84.9
Sometimes, if it was late, I'd close my eyes and nestle against the arm of a sofa—or of a boy, if it was during one of those brief phases I was officially "with" someone—and drift in and out of sleep, letting images of the roads move through my head.
p. 143.5

phases = stages (of the development of something)
It wasn't long after Tommy's third donation, and though he was recovering well, he was still needing a lot of time to rest, and as it turned out, that wasn't a bad way at all for us to start this new phase together.
p. 237.2

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.