If a child or young person goes through a period of strange or difficult behaviour, people
 sometimes describe it as a phase, meaning that it will stop after a while.
 When I was in my early teens I went through a phase of only ever wearing black.
 He started wearing women's clothes but I thought it was just a phase and he'd get
 over it soon.

 The phases of the moon are the regular changes in its shape as it appears to us on Earth.

 If two things are happening in/out of phase they are reaching the same or related stages at the
 same time/at different times.

 phase  verb [T]
 If a new system or plan is phased it is introduced in stages over a particular period of time.
 The minister said that the reduction in armed forces would be phased over the next five
 to ten years.

 If you phase in something you introduce it gradually.
 They will phase the new health care system in over a period of five years.

 If you phase out something you remove it or stop using it gradually.
 They're phasing the larger coin out because it's too heavy.