Someone who is obsessive about audio quality. This includes audio formats, audio equipment, and ripping processes/programs (I.E. Exact Audio Copy). Audiophile's usually have "trained" ears and are able to distinguish the difference between high quality and bad quality music encoding/components through blind testing.
Person: "Hey will you load this CD on to my computer for me while I go grab something to eat?"
Audiophile: "Sure. Do you want it compressed or lossless?"
Audiophile: "What format do you want? ALAC, AIFF, VBR, CBR, AAC, OGG, WMA, WAV?"
Audiophile: "I'll go ahead and rip the CD to FLAC level 8, and you can convert it with dBpoweramp to whatever you want later."