Commonly means happening or appearing frequently, or being widely accepted by most people. When something occurs commonly, it shows up often in everyday life as a regular, expected part of normal experience.

Rain commonly falls in spring. Teachers commonly assign homework on school nights. In conversation, people commonly say “hello” when greeting each other. These are all things you can expect to see or hear regularly, happening repeatedly over time.

The word helps us distinguish between what's typical and what's exceptional. While some students might occasionally forget their lunch, if a particular student commonly forgets it, that means it happens so often it's become their pattern. Similarly, when we say dolphins are commonly found in warm ocean waters, we mean that's where you'll usually see them, as their typical habitat.

Commonly is closely related to usually and typically, though it emphasizes frequency and widespread occurrence rather than just habit. When something happens commonly, many people experience it, not just you. If your teacher says a mistake appears commonly on the test, she means lots of students made the same error.