«الاستعارة هي مجاز علاقته المشابهة»
Isti'ara is a type of majaz whose relation is resemblance.
A word associated with one thing is borrowed for another because the two share a feature. In an ordinary simile, the two terms are typically shown in a relation of comparison; in metaphor, one side is expressed through the name of the other.
Not every figurative expression is a metaphor. Its distinguishing relation is similarity, and the contextual indicator excludes literal interpretation.