A gleam is a bright flash of light. If you were lost on a dark, stormy night, you'd be hugely relieved to see the gleam of a roadside diner's sign up ahead.
Use the verb gleam when something is shining as if it were wet or glinting with light. Your new toothpaste might make your teeth so white that they gleam. You can also describe an emotion that appears briefly as a gleam, like a gleam of hope. And as you hatch a plan to toilet-paper the neighbor's yard, mischief might gleam in your eyes.