A simile says "X is like Y". "Like" or "as" are the words to look for.
I think that the definition needs to be expanded.
First of all, the comparison must be between two essentially dissimilar things. For instance, "The morning sun is shining like a red rubber ball" compares the sun to a red rubber ball, something that you would never mistake the sun for. ("Hey Ralph, look at that red rubber ball coming over the horizon!") The simile is used to give the reader a clearer idea as to some aspect of the compared object. The essential element, however, is their dis
similarity. So, "I am like my twin brother" or "One seagull is pretty much like another" are not
similes even though they fit the quoted definition.
And similes are not limited to comparisons using "like" or "as" - English teachers like to keep it that simple because it's easier to teach it that way, but the definition is broader. To explain, I must first digress . . .
"He has muscles like iron" is a simile. "He has iron muscles" is a metaphor (a compressed comparison). The word "like" (or "as") acts like an equals sign in a math equation. So what would you call the following? "His muscles are stronger than iron."
Think about it a minute before you read on.
By the quoted definition, you could not call it a simile . . . but it is
. The difference between similes and metaphors is that the simile has some word or phrase that acts like an equals sign. "Like" and "as" are the most common, but they're not the only ones.
"Able to leap tall buildings in a single bound (statement of fact), faster than a speeding bullet (simile - not
metaphor), more powerful than a locomotive (also a simile) . . . "
Strictly speaking, I suppose they're all just statements of fact because they are literally true, but if you used "faster than a speeding bullet" about a sprinter, if would be a simile, and if you called a wrestler "more powerful than a locomotive" it would also be a simile.
A final series of examples:
He has muscles like steel. (simile)
He has steel muscles. (metaphor)
He has muscles stronger than steel . . . (simile!)