In the harmony of Western art music a chord is said to be in root position when the tonic note is the lowest in the chord, and the other notes are above it. When the lowest note is not the tonic, the chord is said to be inverted. Chords, having many constituent notes, can have many different inverted positions. Further, a four-note chord can be inverted to four different positions by the same method as triadic inversion. Where guitar chords are concerned the term "inversion" is used slightly differently; to refer to stock fingering "shapes".
http://en.wikipedia.org/wiki/Musical_chord#Inversion
