Simile
An explicit connection between two realities using a comparison word: like, as, similar to.
A simile connects a tenor (what you're talking about) and a vehicle (the image) through an explicit grammatical tool: "like," "as," "similar to." Unlike metaphor, it keeps the two realities distinct — the reader sees the bridge between the banks instead of leaping straight across.
That explicitness makes it gentler and more controllable than metaphor: you guide the reader. It's also its limit — a stock simile ("white as snow") slides by without a trace. A simile's quality almost always lies in the choice of vehicle: concrete, sensory, slightly unexpected.
Example
"O my Luve is like a red, red rose / That's newly sprung in June"
In the workshop
Ban the frozen similes of everyday speech. If your vehicle could appear in an idiom dictionary, find another one.