I just got a weapon that has the following attribute: "4% extra damage against elites".
Does this also apply when I am fighting bosses?
Elite monsters are any monster which have special attributes not normally found on their base type. Champions, Rares, and also Superunique monsters are all considered "elite".
Source: http://www.diablowiki.net/Elite
Well according to the diablo wiki (not sure how "respected" this is) .
A Monster
Elite: Tough. Stronger attacks. Or both.
"Designed to make the player pause and think. High damage attack, high hit points, etc. Something different. Scary and dangerous, more so than the lieutenant. "
And bosses
Bosses (technically known as "SuperUniques") are specially named and located enemies who may have different, unique graphics and special story importance.
This would imply that a boss counts as an elite.