Definitions for "Barre Chord"
A barre chord takes its name from the role of the 1st finger of your left hand. This finger acts as a "bar" across the fingerboard, depressing all six strings and replacing the nut (the ivory piece at the top of the neck). By using your first finger as a "bar," you can move many of the open chords you have learned up and down on the fingerboard.
From the French term barré. The technique of placing the left hand index finger over two to six strings in the fingering of a chord. The great advantage of using barre chords is that they are "moveable shapes" that can be applied at practically any fret.
A guitar chord in which your index finger barres all strings at one fret, and the rest of the chord is built using that fret as the nut. For example, in an F# chord, the index finger barres the second fret, and the other three fingers make an E chord using the second fret as the nut.