Fret

From GuitarWiki

A thin, raised bar of nickel/silver (or stainless steel) wire placed across the fretboard to indicate a specific position of a note, and aid in tuning that note. Most guitars have at least 21 frets, many have up to 24 frets, and some custom guitars will have more. Frets are counted in an ascending manner from the headstock to the bridge, so the first fret is closest to the headstock and represents the lowest notes that can be fingered, the 21st fret and higher are the highest notes that can be fingered. Frets come in a variety of widths from "vintage thin" (6230, which is approx. .080") to "jumbo" (6100, which is approx. .112"). The height of the frets is also something that varies from .036" - .055".

Also "to fret", means to hold down, finger, or "grab" a note or notes with the "fret hand", or the hand on the fretboard -- with or without actually playing the note(s). For example, you may hear someone in a lesson tell the student to "fret the notes of a c major chord". This is an instruction to finger the chord, but not necessarily an instruction to play the chord.