piano chord
A♯dim
A♯1C♯♭3E♭5
Two minor thirds stacked. The fifth is diminished, so it cannot be dropped without losing the chord.
Written the other way round it is B♭dim — the same twelve keys on a piano, a different piece of notation.
What is true of it
A♯dim is A♯, C♯ and E — 1, b3 and b5 counted from A♯.