Accidental means happening by chance rather than on purpose. When you accidentally knock over a glass of milk, you didn't plan to do it: your hand just bumped it while reaching for something else. When a scientist makes an accidental discovery, she finds something important while looking for something completely different.

The word tells us something wasn't intended or planned. If you accidentally insult a friend by making a joke that hurts their feelings, you didn't mean to cause harm. That's different from deliberately saying something mean. The key is that accidental things happen without anyone meaning for them to happen.

Many important inventions came from accidental discoveries. Penicillin, the first antibiotic medicine, was discovered when mold accidentally grew in a scientist's Petri dish. Potato chips were invented when a chef accidentally sliced potatoes too thin. Post-it notes came from glue that was accidentally too weak.

In music, an accidental is a sharp, flat, or natural sign that changes a note's pitch. The common meaning is simply: not on purpose, not planned, happening by chance.