Who’d a thunk spellin’ chords would be so tricky? There’s actually 3 chords we’re talking about.

A diminished chord is a triad and the formula is 1-b3-b5.

Cdim = C – Eb – Gb

http://chord-c.com/guitar-chord/C/diminished/

A diminished 7 chord has 4 notes and the formula is 1-b3-b5-bb7.

Cdim7 = C – Eb – Gb – Bbb

http://chord-c.com/guitar-chord/C/diminished-seventh/

A minor 7 flat 5 chord has 4 notes and the formula is 1-b3-b5-b7. It’s also called a half diminished chord.

Cm7b5 = C – Eb – Gb – Bb

http://chord-c.com/guitar-chord/C/minor-seventh-flat-fifth/