I know this is mostly for Haskell, but I would add (used in various languages):
e: event
k: key in hash map, dictionary, etc.
o: object or reflected object types
v: value in hash map, dictionary, etc.
t, u, v, w: vectors (borrowed from math)
Would be fun to think about this for a while and come up with a more serious dictionary with examples and links to code.
