A zero fret is a piece of fretwire mounted into a stringed instrument fretboard, which is installed in place of a nut. The zero fret can be either the same size as the rest of the fretwire on the fretboard, or it can be a larger size.
The reason for using a zero fret is that it makes getting the string action just right a lot easier than using a standard nut. Because a zero fret only performs one of the roles of a traditional nut (setting the string height) some other method is needed to position the strings.
Synonyms:
zero frets