Mittelschmerz is a German word that translates literally to "middle pain". Also known as "ovulation pain".
It is characterised by lower abdominal and pelvic pain that occurs roughly midcycle in a woman's menstrual cycle. It is thought that bleeding associated with rupture of a follicular cyst causes this by irritation of the peritoneum. The pain subsides within hours and may be accompanied by a small amount of vaginal bleeding.
Some women have mittelschmerz regularly and can time their ovulation by it but many never experience it. In theory, going on the pill (oral contraceptive pill) should remove this pain altogether as it prevents ovulation.
Mittelschmerz is sometimes mistaken for appendicitis and is one of the differential diagnoses for appendicitis in a woman of child-bearing age.