A cliché is an overused phrase or idea that has lost its originality and impact through repetition. When someone says “only time will tell” or describes something as “a blessing in disguise,” they're using clichés: expressions that were once fresh and clever but have been repeated so many times that they've become stale and predictable.

Writers and speakers usually try to avoid clichés because they can make communication feel lazy or unimaginative. Instead of writing “it was raining cats and dogs,” a good writer might describe exactly how the rain looked and sounded. Instead of saying a character is “as strong as an ox,” they might show the character's strength through specific actions.

Clichés aren't always wrong to use. In casual conversation, phrases like “better late than never” communicate ideas quickly and clearly. But when you're trying to express something meaningful or creative, reaching for a cliché is like serving yesterday's reheated leftovers when you could make something fresh. Your English teacher probably circles clichés in your essays not to be picky, but to encourage you to find your own original way of expressing ideas.