Honestly, it's a pretty loose definition.  A smash is a subset of julep (which is also loosely alcohol, sugar, water, and shaved ice).  So a smash falls along this line with (typically) a spirit, simple syrup, ice, and some kind of fruit...served in a shorter glass than the traditional julep.
It's seasonally flexible, incorporating both fruit and herbs as needed, and sometimes people top it with seltzer to water it down a bit.  It's refreshing and perfect for summer.