How do you define a "constant" vs "seasonal" friend? 

I think the concept of a constant and seasonal friend is about the duration, intensity and depth of the relationship and interactions.

A “seasonal” friend might be a school mate with whom you hung out intensely for the season of school as you shared similar routines and interests but did not continue to meet (as frequently) beyond your schooling years and became unaware of what was happening in their lives.

A “constant” friend might be a friend whom you hung out with intensely for the season of school and continued building the relationship with after leaving school. Inevitably, the dynamics of this friendship would have changed. Perhaps both of you are meeting less, but still aware of what’s happening in each other’s lives, and catch up occasionally over deep conversations. This is a friendship that remains but changes throughout the different seasons. I’m not going to romanticise the idea of a constant friend. Everyone disappoints at some point, so manage your expectations and the pace of your friendships :)