Tertium comparationis

Tertium comparationis (Latin for "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. The New Testament scholar, Adolf Jülicher, applied the concept of tertium comparationis to the parables of Jesus. According to Jülicher, a parable or similitude (extended simile or metaphor) has three parts: a picture part ("Bildhälfte"), a reality part ("Sachhälfte"), and the point of comparison ("tertium comparationis") between the picture part and the reality part. "The kingdom of heaven is like treasure hidden in a field, which someone found and hid; then in his joy he goes and sells all that he has and buys that field" (Matthew 13:44). In this parable, the picture part is the hidden treasure, the reality part is God's kingdom, and the "tertium comparationis" is the inestimable value of the kingdom.