Neil.

Aside from the root tone of a chord, I have heard the term root used to describe the basic 3 tone chord upon which the more complex chords being discussed above are built. Mostly they refer whether the 'root' chord is a major or minor triad. For example, the 'root' of a D7sus4 is the D major chord containing the notes D-F#-A. Building the D7sus4 adds a fourth and a flat (or dominant) 7th to 'root' chord, yeilding D-F#-A-G-C. Similarly an Amin7 is based on the A minor root of A-C-E, and is consructed by adding the dominant or flat 7th (A-C-E-G).

Diminished chords (where all intervals are minor thirds) and augmented chords (where all intervals are forths) might be considered 'root' chord forms, but someone else would have to verify that for me.

Occasionally I have heard the resolving chord in a progression refered to as the root, e.g. the C chord when playing in the key of C. Its the key you typically return to complete/resolve the progression. But this usage may be incorrect, as the technical name for such a chord is the TONIC.

Best Regards.