Tertium comparationis (Latin = the third [part] of the comparison) is the quality that two things which are being compared have in common. It is the point of comparison which prompted the author of the comparison in question to liken someone or something to someone or something else in the first place.
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. However, they must possess at least one quality in common. This common quality has traditionally been referred to as tertium comparationis.
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.