In medieval England, Leyrwite was a fine levied for engaging in sexual activity that had been banned by the Church. It appears to have been exclusively English; it does not seem to have been practiced in any other part of Britain, and there's no evidence for it anywhere in Europe. It was also not enforced uniformly throughout England, and where it was enforced, the specifics often varied from one manor to another.
Leyrwite was only levied on unfree peasants. The most common transgression against which it was levied was sexual intercourse out of wedlock. Usually, the only time authorities could be certain such had taken place was when an unmarried woman gave birth to a child, but other transgressions were also prosecuted. Leyrwite was almost exclusively levied against women.
The fine of Leyrwite was more frequently enforced during times of economic hardship, when revenue was slight. Still, it appears to have been more a form of social control than a financial tool. Fines were traditionally as low as sixpence or so, although for a poor woman, such an amount could be a difficult onus. If a woman and her sexual partner had entered into a contract to marry, it could obviate the fine; but such was not always the case.
A woman who paid Leyrwite could marry in the future without any restrictions. Like merchet, the enforcement of Leyrwite began to decline in the 15th century.