TMCS1100-Q1: What exactly does quiescent mean for this device?

Luke,

Quiescent current is the standard definition here. This current is the current drawn by the Vs pin under a no load condition, which means the totality of the part is energized. Another way to think of this is as the "overhead" of the device, or the current required to properly energize the device for use before any load is applied which might further increase this value.