A diminished 7th has these notes: A, C, Eb, Gb.
You can also take any of the chord notes to be the root note. So we can also call this chord:
A diminished 7th
C diminished 7th
Eb diminished 7th
Gb diminished 7th
Diminished 7th chords are chord vii in minor keys but are usually played as passing chords between one chord and another, for example:
Cmajor7 / C# diminished 7th / Dm7 / G7 / Cmajor7
A diminished 7th can also be thought of as Ab7b9.
Tune in tomorrow for another guitar chord of the day!