Bringing two different things together with an explicit word, such as "like", "just like" or "seems". Unlike the metaphor, the comparison makes visible that there are two things.
In Notes from Underground
Then know that I am ashamed, and how ashamed I am, that above all else I dread it, even more than if I had stolen, because I am vain as though my skin had been stripped off, and even the air hurts me.
Vanity is compared to skin torn off, with the «as though» linking the two ideas: he feels even the air as pain. The physical image gives weight to a flaw that, stated abstractly, might sound light.