What makes a razor a rattler?

I've seen this term all over this forum since I've started reading it, and I haven't quite figured it out yet. Time to consult the SRP collective mind. Please forgive me if it's in a FAQ or sticky somewhere.

Is it the grind? Is it the blade profile? Is it the sound it makes while stropping (or shaving)? Is it just because someone etched "R-A-T-T-L-E-R" on it??

Is a Barber's rattler different from a "plain old rattler"?

A short explanation or a point to a thread that talks about it is appreciated - thanks!