amp = amplifier (typically a headphone amplifier, in these forums)
IC = interconnect
DI/O = digital input/output, commonly refers to the Art DI/O
FR = requency response
Ety = Etymotic Research
hp = homepage, hewlett packard, but in this forum, headphones!
caps = capacitors
cmoy = a class of basic amplification circuit proposed by Chu Moy
M³ = an up-and-coming DIY amp being designed by Morsel and AMB. Possible successor to PPA
PTH = PPL's Tube Head, another up-and-coming DIY amp designed by PPL. (yes, I made this one on the fly)
And some people:
Jan = Jan Meier, of Meier Audio.
Todd = Todd Green, of Todd the Vinyl Junkie
Tyll = Tyll Hertsens, of HeadRoom