Metaphor

A figure of speech in which a word or phrase that usually designates one thing is used to designate another, thus making an implicit comparison...

"the dawn of civilization"

"this student will blossom in college"

"a sea of words"

or an explicit analogy.

"like a ship at sea"

"as endearing as a warm puppy"

From the Greek meta (across) and pherein (to bear).

A metaphor is a load-bearing word.