Bringing two different things together with an explicit word, such as "like", "just as" or "seems". Unlike metaphor, comparison makes it plain that they are two things.
In The Brothers Karamazov
Dreamy love craves a quick deed, soon satisfied, and for everyone to look at it. One even gives one's life, provided it does not last long, is accomplished at once, as on a stage, and everyone looks on and praises. Active love, on the contrary, is work and perseverance, and for some, perhaps, a whole science.
Giving one's life "as on a stage" compares the sacrifice of dreamy love to a performance. The image shows that the grand gesture looks for an audience and praise, and that for this reason it is not love.
Book II: An untimely meeting, Elder Zosima to Madame Khokhlakova