False labor: Intermittent non-productive muscular contractions of the womb (uterus) during pregnancy, most commonly in the last two months before full term. These contractions are non-productive in the sense that they do not produce any flattening (effacement) or dilation (opening up) of the cervix.
It is often said that false contractions are for "practice," for getting the woman ready for true labor, but this is pure speculation since there is no convincing evidence that these contractions aid in the delivery of the baby in any way.
An episode of false labor can surprise and concern a pregnant woman, especially when she experiences these contractions for the first time. But, by definition, these contractions are of limited duration and do not indicate the onset of active labor.