From Sept 2011
It has been suggested that individual amplification is a boundary line for whether something is a choir or not. I actually think this is a good general litmus test.
However, it takes but 2 seconds on YouTube to find examples that break the rule:
Definitely a choir, and every member has individual amplification (due to the nature of the venue and the fact that they were filming, so they needed pristine sound).
and this is definitely an acappella group, but singing completely unamplified:
Obviously the rule must be "I know it when I hear it."