Simile

A simile is a figure of speech that expresses a resemblance between things of different kinds, usually formed with 'as' or 'like'. Some examples of well know similes include ‘ as busy as a bee’, ‘as thick as two short planks’, ‘as easy as falling off a log’ and, of course, ‘as mad as a box of frogs’.

The word simile comes from the Latin similis (like).

Here are a few less well know / freshly minted ones courtesy of Simon:

as dextrous as an octopus using seven pairs of scissors
as reliable as a fox wearing a cravat
as doomed to failure as a bouncy castle birthday party for hedgehogs
(the above suffer from not being pithy which, in my view, is one of the essential elements of a good simile)
as easy as herding cats
as spotty as a teenage leopard
like a duck to custard
like lambs to the mint sauce
as big as a very big thing