Audiophile slang
Synergy
Also called system synergy, pairing
Synergy is the claim that certain combinations of gear sound better together than their parts suggest: one amplifier with one headphone, say. Some of it is real electrical interaction, some is taste, and some is a way of justifying a purchase.
The measurable part is interaction. A tube amplifier with a high Output impedance changes the tonal balance of a low-impedance headphone; a low-Sensitivity planar opens up with an amplifier that has power to spare; a Bright headphone and a Warm source can land somewhere pleasant. Those pairings have explanations.
The rest is harder to pin down, and objectivists regard "synergy" as the word people use when two transparent components are said to sound different together. Listeners disagree. The practical advice both sides give is the same: match power and impedance first, then trust your ears.