Tertium comparationis (Latin for "the third [part] of the comparison") is the quality that two things which are being compared have in common.
If a comparison visualizes an action, state, quality, object, or a person by means of a parallel which is drawn to a different entity, the two things which are being compared do not necessarily have to be identical.
The most common devices used to achieve this are metaphors and similes, especially, but by no means exclusively, in poetic language.
In many cases one aspect of the comparison is implied rather than made explicit.
The New Testament scholar, Adolf Jülicher, applied the concept of tertium comparationis to the parables of Jesus.