To mar means to damage or spoil something's appearance or quality. When you accidentally scratch your new bike, the scratch mars its shiny surface. When someone's rude comment mars an otherwise perfect birthday party, it puts a damper on what should have been pure fun.

The word suggests damage that isn't complete destruction but still matters. A single spelling error can mar an excellent essay. A stain on your favorite shirt mars its look. Rain might mar your plans for an outdoor picnic. The damage might be small, but it's noticeable enough to bother you.

Mar often appears when something was going well until one thing went wrong. You might say, “Don't let one bad grade mar your whole report card,” or “A few weeds marred the otherwise beautiful garden.” The word carries a sense of disappointment because whatever got marred was good before the damage happened.

The past tense is marred. People also use the phrase without a mar to mean something stayed perfect and completely undamaged.