(the use of) an expression comparing one thing with another; the use of ‘as’ or ‘like’
She described her child’s dirty face using the simile “as black as coal”. [C]
The lines ‘She walks in beauty, like the night…’ from Byron’s poem contain a simile. [C]
The poem was rich in simile. [U]
Basically, we use the expression as + adjective/adverb + as when we want to say that two things are the same in some way:
Do you think they are as good as they say?
I’m just as capable as you are – let me do it!
The weather’s as miserable as usual.
Note that we can also use as + adjective/adverb + as (or so + adjective/adverb + as) after the word not in negative expressions:
They are not as/so good as they say they are.
You’re not as/so capable as I am – let me do it!
The weather’s not as/so miserable as usual.
Similes with as + adjective/adverb + as are used in many “set” expressions commonly used by native speakers when comparing things:
Some of these monuments are as old as the hills.
The floor’s as clean as a whistle.
Little Jane’s as quiet as a mouse – I wonder what she’s up to.
That shower’s made me feel much better – as fresh as a daisy.
As a learner of English, you might not feel very comfortable using this type of expression. In this case, you should feel free to invent your own similes, as long as the comparisons are valid ones. For example, English speakers say “as white as a sheet” because, traditionally anyway, sheets were usually white. A learner might prefer “as white as snow”, or “as white as cotton wool”, and these are good “invented similes” because the image in people’s minds of snow and cotton wool is of whiteness.