Diminished chords are a source of some confusion. The chord that
you generally come across includes the intervals
unison, minor third, diminished 5th and diminished 7th.
Sometimes this is denoted Xdim7 (eg Adim7) and sometimes not. People who differentiate the chord by denoting the 7th generally use the plain Xdim to mean the 3-note triad.
And to make it more confusing others will use the 6th instead of the diminished 7th - this makes some chords more palatable (Cdim would be C Eb Gb A).