The smoothest amp you've heard?

I don't understand what smooth even means?

Perhaps others who have posted above can help me with my (lack of) understanding?

@jakesnack  @david_ten  Read my first post in this thread. The explanation is in the first sentence.

I comprehend the sentence and premise. Its a very good technical explanation. However, the point I was attempting to make was, from a sonic perspective "smooth" doesn't present itself as something I am attempting to ascertain, or looking for in a amp? I have listened to many amps, I never recall saying "that amp is so smooth". I'm not trying to sound disrespectful. I think its a useless adjective when evaluating sonics of a component. Do you think a sound engineer says "we have to do it again, not smooth enough" IMO, Its a pointless term. 

Some of the First Watt amps , F3 comes to mind.

It doesn't get any SMOOTHER than this:

https://www.youtube.com/watch?v=6Whgn_iE5uc

Adg audion amps are very smooth