"Snug as a bug in a rug," e.g.
I believe the answer is:
simile
(Other definitions for simile that I've seen before include "The likening of one thing to another" , "Figure of speech used in making comparisons" , "Figure of speech expressing resemblance between different things" , "Lies I'm finding in figure of speech" , "Most common figure of speech, one of comparison" .)