An abnormal period simply means a not normal period, for instance, say your period is usually light every month and then one month you have a really heavy period that would be abnormal. Also, if your period always comes on or around the same day, and then one month is comes 1 or 2 weeks early or late then that would be considered abnormal. Or if it last longer or shorter then usual that also would be abnormal. It is just a period that is not the same as it should be like every other month. Hope this helps.