A wrinkle is a small fold, crease, or line that appears in something that was once smooth. Your clothes get wrinkles when they've been wadded up in a backpack or left in the dryer too long, forming those annoying creases that make a shirt look rumpled instead of neat. Skin develops wrinkles as people age, especially around the eyes and mouth, creating lines that show where someone has smiled or squinted thousands of times over the years.

You can iron out wrinkles in fabric by using heat to smooth them flat. The phrase “ironing out the wrinkles” also means fixing small problems in a plan: if your class is organizing a field trip, you might need to iron out a few wrinkles in the schedule before everything runs smoothly.

As a verb, to wrinkle means to form wrinkles or to cause something to become wrinkled.

A wrinkle can also mean an unexpected complication or clever twist. If your teacher announces a surprise wrinkle in the rules for a project, she's adding something that changes things in an interesting way. When you read a mystery novel and discover a new wrinkle in the plot, you've found a surprising detail that makes the story more complex and intriguing.