The alignment of one thing's frequency with another, producing recognition without imposition. Not matching and not mirroring — two frequencies acknowledging each other without either having to change.
Latin resonantia (echo), from resonare (to sound again, resound): re- (again) + sonare (to sound), from sonus (sound). In physics since the 17th century: the amplification that occurs when a system is driven at its own natural frequency — a small repeated push producing a large response, because the push matches what the body already wants to do.
The house's own first word, and the first atom of nearly every molecule in it. The physics is the point: resonance is not force but MATCH — the right frequency, gently and repeatedly applied, moves what force cannot. Every realm named Resonance-something is a claim that the thing works with a person's own nature rather than against it.