In music, a diminished triad (also known as the minor flatted fifth[citation needed]) is a triad consisting of two minor thirds above the root. It is a minor triad with a lowered (flattened) fifth. When using popular-music symbols, it is indicated by the symbols “dim”, “o”, “m♭5”, or “MI(♭5)”. For example, the diminished triad built on C, written as Co, has pitches C–E♭–G♭:

Source: Diminished triad – Wikipedia