Inversion usually refers to the lower-most note of a chord. If the root is the lower-most note, the chord is said to be in root structure. If the 3rd is the lower-most note, it is said to be in ‘first inversion’, i.e. the root has been placed in an upper voice leaving the 3rd as the lower-most note (voice), and is traditionally indicated ‘6’ because of the interval from the 3rd to the root. See ‘figured bass’.