Simple Definition:

A simile is a figure of speech [An expression that is not meant to be taken literally] that compares two things using the words “like” or “as.” It is often used to create a vivid image in the reader or listener’s mind. For instance, in the sentence “Her hair was as bright as the sun,” the use of “as” makes it a simile.

Very Simple Definition:

A simile is a way of comparing two things by using the words “like” or “as”.
Example: The stars were shining like diamonds in the sky.