Strictly speaking, a chord can't be "augmented" or "diminished".
A chord can only be "major" or "minor". The interval between 1 and 3 give the nature:
("T" = "Tone", "ST" = "Semi-tone")
- 1T1/2 between 1 and 3 implies "minor"
- 2T between 1 and 3 implies "major"
All of the other terms — augmented, diminished, etc. — require to define which interval is augmented, diminished, etc. Exception made for some of them (see below).
'augmented' means one semi-tone (ST) over the perfect state.
'diminished' means one semi-tone (ST) under the perfect state or the minor state.
On 6 degrees, the 5th (quint) is perfect. That means 3T1/2 from 1 to 5.
So the perfect state, 5th can be 'augmented' (3T1/2 + 1ST) or 'diminished' (3T1/2 - 1ST).
The same for the 4th (quart), its mirror interval : 2T1/2 is a perfect 4th. 2T1/2 + 1ST is an augmented 4th, 2T1/2 - 1ST is a diminished 4th.
We call such a chord a chord with augmented 5th (C aug.5th) or a chord with dinunished 4th (F dim.4th).
Sometime, we talk about de "augmented chord" for "chord with an augmented quint", but it's an « artistic licence», an abuse of language ;-). We do so because the 5th is the very most augmented interval, in a chord.
Like 3th which can't be perfect, the 7th can be minor (C-Bb) or major (C-B). Unlike the 3th, the 7th can also be diminished (F#-Eb).
So, you see, we (always) have to know which degree is altered in the chord to qualify it.
Exception made for:
"dim7", where we only talk about 7th. You have to know that in this chord the 3th is always minor and the 5th is always diminished. No exception. It's a so frequent chord that we have to give it a shortname, a pseudo.
"semi-dim7" (used in english? in french: 7e demi-diminuée). You have to know that 3th is always minor and 7 is always diminished.
I'd like to add:
- sus (Gsus7, ), where the 3th is replaced with the 4th (G C D F -> 1 4 5 7)
(If someone wants to correct my very bad english, I give him white card ;-)