Punctuation

"Period" is the name of the punctuation mark used to indicate the end of a sentence. (there! see that one! (these last two were exclamation points, though))

Menstruation

"Period" is a slang term, or euphemism, for the part of a woman's menstrual cycle where she is bleeding.

Society for Creative Anachronism

In the SCA, "period" refers to the time period that is studied by the members ("pre-17th Century," to be specific).

Cycles

"Period" is the amount of time it takes an object in orbit to return to its starting location. Also used with pendulums and other things with cycles, such as economics.