Bringing two different things together with an explicit word, such as "like," "just as" or "seems." Unlike metaphor, the comparison makes clear that they are two things.
In The Magic Mountain
a fever of matter that accompanied the process of incessant decomposition and reconstitution of albumen molecules [...]. It was neither matter nor spirit. It was something between the two, a phenomenon sustained by matter, just like the rainbow over the waterfall, and equal to the flame. [...] Illness was the licentious form of life.
Life is compared to "the rainbow over the waterfall" and to "the flame": something sustained by matter but not reduced to it. The phrase "just like" lets both things be seen side by side and explains what lies between matter and spirit.