I wish I had better terminology to describe relationships with people. Putting aside for the moment the differences between best friends, good friends, friends, light friends, favored acquaintances, acquaintances, nemeses, and so forth, there's still the matter of rate of change. For example, you could have recently met someone and want to spend all of your time with them but they wouldn't be your best friend, or you could have a long-time best friend with whom your relationship is very stable, or you could have an old flame with whom your friendship is starting to dwindle. I guess our only option is to express levels of friendship as equations over the variable t. f(t) would be defined for all t >= 0, and would represent your level of friendship on a scale of -10 (archnemesis) to +10 (best friend) at a point in time t weeks from now. So, if you just met someone the night before and fell madly in love, you could say that they were a "5 + 5t" friend, and that best friend from middle school that you haven't talked to in half a decade could be a "7 - 0.08t" friend. I expect that people will convert to using this system at the same time that they start fully parenthesizing their written communications and start using the metric system.
(Of course, the linear equations are meant only for informal communication. In formal communication, such as love letters, one should probably present something a bit more polynomial that will actually become asymptotic on the axis f = 10.)
All ridiculousness aside, though, I really do wish I had more descriptive terminology to use to talk and write about the various people that matter in my life. Basically right now I have to use "friend," "good friend," or if they don't qualify for either of those, some other vague word that sort of describes them but not the actual state of our relationship.