[junkman]

Hiya, ok let's say you had a two note major chord - root and third. Let's say C and E. Then say you added an A below C to turn it into a minor chord - what is the technical name of that A note? Is it just "the root of the relative minor" or does it have a snappier name more akin to "sub-dominant" and the like?

Thank you!

[Ramshackles]

In the key of C major, an A-minor chord would be the submediant.

[Ramshackles]

but of course, it means nothing without the context of the key.
I.e in your example above, you could have stated that the key is A minor...in which case it is the tonic 

[Boydie]

RAM has nailed this one for you 

To add a little more info...

As RAM has said it depends on the context in which you are naming that "A" note

If it is in relation to the degree of the Major scale (in this case C Major) the A note is the sixth note (or degree) of the major scale

The snappier name for the sixth note is called the "submediant" as it is in the middle of the "upper tonic" (the C at the end of the scale) and the "subdominant" (the fourth note of the scale, which in C Major is the F)

However, in the context of the A Minor chord the A would simply be the root note

Finally, as RAM has pointed out, if the key (tonal centre) is A minor (implied by the choice and order of chords along with how the melody is constructed and resolved) then the A note becomes the tonic note of A Minor

[junkman]

Ok great, thanks for your help guys. Yes the idea was you're in C major and hearing that chord, and then the A note is introduced, so it sounds like "submediant" is the one.