Irregular means not following a normal pattern or not happening at predictable times. If your friend usually walks to school but some days gets a ride, comes on a bike, or stays home, you could say she has an irregular schedule. When something is irregular, you can't count on it being the same way each time.

The word appears everywhere once you start noticing it. In math, an irregular shape doesn't have equal sides or angles like a square does: a random blob is irregular. In grammar, irregular verbs don't follow the normal rules for past tense: instead of “walk/walked,” you get “go/went” or “eat/ate.” Doctors might detect an irregular heartbeat when the heart doesn't beat in a steady rhythm. A basketball that bounces in unpredictable directions has an irregular bounce.

The opposite of irregular is regular, which means following a predictable pattern. Regular trains arrive on schedule. Regular polygons have all equal sides. When your dentist asks if you brush regularly, she wants to know if you do it consistently, not just occasionally.

People sometimes use irregular to describe things that don't meet expected standards, like irregular merchandise at a clothing store (items with minor flaws sold at a discount). The key idea is always the same: something that doesn't match the usual pattern or expected form.