Tritone

Six semitones. Because it splits the octave in half, the tritone is its own mirror image: invert it and you get another tritone. That symmetry is why it gives your ear no sense of direction on its own, and why it needs the chord around it to decide where it goes.

Strained and hollow, and impossible to mistake for anything settled. Once you have heard it inside a dominant seventh you will recognise it everywhere.

The interesting part is that it can resolve two ways. Written as an augmented fourth it tends to expand outward; written as a diminished fifth it tends to contract inward. Same keys, different spelling, different expectation.

Between the third and the seventh of every dominant seventh chord. In the diminished triad, as the outer interval. In tritone substitution, where two dominant chords a tritone apart share the same pair of notes and can stand in for each other.

Nothing, really. Once you know it you know it. The confusion is in the naming: augmented fourth and diminished fifth are the same distance on a keyboard and different notes on paper.