The word period has several meanings:

1. A length of time with particular characteristics or events. A period of rain might last several days. History teachers divide the past into different periods: the Colonial period, the Renaissance period, the Ice Age. Your school day is divided into periods for different subjects. When someone says “we're going through a difficult period,” they mean a stretch of time when things are harder than usual.
2. The punctuation mark (.) that ends a sentence. You put a period at the end of a statement to show the thought is complete. People sometimes say “period” out loud after a statement to emphasize they're completely serious and won't change their mind: “You need to finish your homework. Period.”

In science class, you might learn about the periodic table of elements, which organizes chemicals in repeating patterns. Something that happens periodically occurs at regular intervals, like a newsletter that arrives periodically throughout the year.