Tube Discussion Microphonics?

I have read in quite a few places where someone complained of this tube or that being microphonic. I understand what is meant in a discussion of matching output parameters but I have heard people say that a pair, or quad, of tubes were matched but still microphonic.

What does it mean for a tube to microphonic? How does this translate into audible identification? I mean how do you tell by listening whether a tube is microphonic or is just not a good sounding tube or not good sounding with a particular component?