Modern Definition
A pagan marriage-type commitment. Not necessarily legalized and usually for a specific period of time - most often the traditional "year and a day."

Traditional Definition
Handfastings were popular in Europe during the Middle Ages. A couple would be handfasted for a year and a day before being formally wedded in the local Catholic Church. The handfasting was intended to assure the fertility of the couple in a time when it was imperative to have as many children as possible.