Bringing two different things together with an explicit word, such as "like," "just as," or "seems." Unlike metaphor, the comparison leaves visible that there are two things.
In The Master and Margarita
Love appeared before us, like a murderer who comes out of nowhere in a side street, and hit us squarely.
Love is brought close to "a murderer who comes out of nowhere in a side street" through the word "like." The ambush image strips the declaration of its sweetness: the feeling comes from outside, without warning, and in the Master's words, it "hit us squarely."