semitone
The smallest step on the piano roll: one row up or down. Twelve make an octave.
The smallest step in Western music: one row up or down on the piano roll, and twelve of them make an octave. Every interval is just a count of semitones. On a keyboard it is any key to its nearest neighbor, black or white.
Learn it: Intervals · The piano roll
The one-sentence definition at the top is the same one the Theory Aide extension shows as a tooltip inside Ableton Live. Hover any dotted term for its own.