It's a shorter way of saying "menstrual period" as opposed to other kinds of periods, such as orbital periods, periods of time, etc. The etymology of the word (Greek) is something along the lines of...
Sometimes during ovulation, (the phase that is in between each period) your hormones can run high and you may experience symptoms similar to that of your period, such as cramps and irritability. If...
If you're on birth control and have missed any pills this can result in a early period. Alternatively pregnancy, irregular periods or hormone imbalance can also cause a early period.