A heyday is the period when someone or something is at their peak of success, power, or popularity. It's the golden time when everything seems to be going right and achievements come easily.

The Roman Empire had its heyday when it controlled most of Europe, North Africa, and the Middle East. Baseball legend Babe Ruth was in his heyday during the 1920s, when he dominated the sport and became a national hero. Your grandmother might tell you about the heyday of drive-in movie theaters, when families would pack into cars on summer nights to watch films under the stars.

The word often appears when people look back at a time that has passed. A retired athlete might reflect on her heyday when she won championship after championship. An old factory town might have seen its heyday decades ago, when thousands of workers filled its streets.

The phrase “in one's heyday” suggests a specific window of time, not forever. Everyone and everything has a heyday: musicians, inventions, sports teams, even trends and fashions. People often use the word to appreciate great moments while they last and remember them fondly after they've passed.