What exactly is meant by the term? I've seen it several times but I don't know what it means. I've been playing guitar for years but for some reason I've never learned that term.
It could be. The word 'tremelo' means rapidly changing volume just as 'vibrato' means 'rapidly changing pitch. Whether it's muted or not depends on the song, but either way, if it's too fast or arhythmical to be conventionally notated, you'd consider it tremelo picking.
Just listen to any black metal band and 90% of the riffs will be tremolo picked. Off the top of my head, good examples of tremolo picking would be the first riffs of "Ea, Lord of the Depths" by Burzum or "I am the Black Wizards" by Emperor or "Drowned" by Entombed as used in a more death metal context.
Temolo picking owns. It sounds really cool. But I don't like the way it sounds when a power chored is temolo picked, which you sometimes find in black metal.