Freyleks is a yiddish term, popularly used to refer to a joyful Niggun, a form of Jewish religious song or tune sung by groups. However, some researchers see it as a "Terminus Technicus," a professional-scientific term that, in this case, signifies a musical genre. It seems that Freylekhs tunes comprised the core of the traditional Klezmer repertoire in Eastern Europe, and were the most popular amongst the Dance tunes.

Source: Jewish Music Research Centre