Erratic means unpredictable and inconsistent, changing randomly without a clear pattern. When something is erratic, you can't count on it to behave the same way twice.

A basketball player having an erratic game might score three baskets in a row, then miss ten shots, then suddenly make another two. A friend with erratic behavior might be cheerful one moment and grumpy the next, making it hard to know what to expect. Erratic weather bounces between sunny and stormy without warning.

The word often suggests something isn't working properly. A car driving erratically might swerve, speed up, slow down, and change lanes unpredictably, making it dangerous for other drivers. A computer running erratically crashes at random times, making it unreliable for important work.

Erratic is different from simply being unpredictable: it emphasizes the irregular, inconsistent nature of the changes. A surprise party is unpredictable, but it's not erratic because it follows a plan. Erratic behavior has no plan or pattern. When a student's test scores are erratic, jumping from 95 to 65 to 88 to 55, teachers wonder what's causing the instability. Consistency and reliability are the opposite of erratic.