C “diminished”. The abbreviation dim or dim7 (respectively ° or °7) stands for a diminshed or diminished 7th chord. To get a diminished chord you can take a minor chord and lower the 5th by a half step (1 fret) to a diminished 5th (b5, „flat 5“). Alternatively you can stack two minor thirds on top of the root: e.g. root C (1), minor 3rd Eb (m3) and another minor 3rd on top of that is Gb (b5).
In practise the diminished chord is usually played with a diminished 7th. So if a chord is just named dim, you can read it as dim7. The diminished 7th (dim7) again is a minor 3rd away from the b5. Consequently a Cdim is normally played with the 4 chords: C (1), Eb (m3), Gb (b5) and Bbb (dim7).
The diminished 7th Bbb is effectively the same tone as the 6th A.
Since the dim7 chord is based on just minor 3rds, it is completely symmetric. As a result the four chords Cdim7, Ebdim7, Gbdim7 and Adim7 have exactly the same chord tones. You can normally play all of them where only one of them is written.
The diminished chord mostly occurs as passing chord between two chords a whole-step away, e.g. Bmaj7 1dim7 Dbm7 Ddim7 Ebm7 Ab7(b9) Dbm7 Gb13 B6/9 …
In the blues form (especially the modified jazz blues) a diminished chord is often added a half-step above the IV7 chord. In a blues in F:
| F7 | Bb7 Bdim7 | F7 | |
| Bb7 | Bdim7 | F7 | | …