A scar is a place where a wound healed but is still visible, or if you're little, it's a mark where a boo-boo was.
Scar is from the Greek word eskhara, a "scab formed after a burn." It's all that and more. Scar can be a noun or a verb — you can have a scar on your knee after your skateboarding incident, and a bad round of chicken pox can scar you. Something scary can scar, you, too, like a horror movie or catching your parents in the act. Scars can be emotional. Take one letter off scare and you're left with a scar.