When something happens accidentally, it happens by chance or mistake rather than on purpose. If you accidentally knock over your glass of milk at breakfast, you didn't mean to do it. Your arm just bumped it while reaching for the cereal. If you accidentally step on someone's foot in a crowded hallway, you weren't trying to hurt them; it just happened in the chaos of everyone moving around.

The word describes actions that weren't planned or intended. Maybe you accidentally grabbed your sister's backpack instead of your own because they look similar. Or perhaps you accidentally deleted a file on the computer when you meant to click something else.

Accidentally is the opposite of deliberately or intentionally, words that describe doing something on purpose. When you apologize for something you did accidentally, you're letting the other person know you didn't mean any harm. Understanding this difference matters: bumping into someone accidentally deserves an apology, but doing it deliberately is much more serious because it shows you chose to cause a problem.