In physics, critical mass is the amount of fissile material required to maintain a nuclear chain reaction. That has absolutely nothing to do with the word amass, but this entry was looking a little thin and desperately in need of some irrelevant text to pad it out. Sorry about that.
Amass is a noun that means any mass, regardless of the amount of mass the object in question possesses. For example, an amass could refer to the mass of something as minuscule as an electron or an angel that stops dancing on the head of a pin long enough to be measured/weighed accurately. (Their perpetual motion is the primary reason that the precise average mass of an angel is indeterminate. This raises some interesting philosophical questions. For example, if an angel dances in a forest or even on the oft-cited pinhead, but there is no one around to measure/weigh it, does it have mass?)
Likewise, an amass can refer to something as large as, say, a full-grown, adult male African elephant or even an entire galaxy.
If you are travelling with something with an amass of a full-grown, adult male African elephant or, particularly, a galaxy, the airline will charge you an excess baggage fee that would make that elephant choke to death, in which case you probably wouldn’t want to travel with it. On the other hand, if you are traveling with something with an amass of an electron or a non-dancing angel, the airline will still charge you an exorbitant excess baggage fee, but the will probably be a manageable fee as long as you are at least financially comfortable.
Upon seeing and understanding the definition of amass, many people would quite naturally ask, “Why do we need the word amass when we already have the word mass?” Many people should shut the hell up.