A guitar or other stringed instrument chord shape or fingering that can be moved to various positions on the fretboard producing the same chord type (major, minor, 7th, 9th, et al) but in a different key/with a different root. Any chord shape that does not use open (unfretted) strings qualifies as a movable chord.
A prime example of a movable chord is a barre chord (though a movable chord does not necessarily have to be a barre chord): on a standard tuned guitar, a major barre chord shape at the third fret products a G major chord. Moving the entire shape up two frets to the fifth fret products an A major chord. Moving the chord shape up another two frets, to the seventh fret produces a B major chord, and so on.