Note weight is the perceived thickness, body, density, or heft of individual notes — how substantial each sound feels. It's one of the most useful single concepts for decoding a whole family of descriptors, because so many of them are really just note weight under another name. A piano strike that lands solid and authoritative has it; one that sounds papery and insubstantial lacks it.
The family runs from light to heavy. Thin or lean is light note weight — notes feel wispy and insubstantial, sometimes sterile or clinical, the lower mids underfed and the sound a little dry. Thick, full, or weighty is heavy note weight, notes dense and substantial with real body. Lush is thick note weight presented sweetly, with a touch of smoothness and warmth. Warm is strongly correlated, since both tend to come from a lift in the lower midrange and mid-bass — warmth is the tonal tilt, weight is the resulting body of the notes. And natural is note weight that matches what real instruments and voices have, neither padded nor skeletal.
It's largely a frequency-response phenomenon. Emphasis in the lower midrange — very roughly 250 to 750 Hz — and the mid-bass lends body, warmth, and heft; recession in that region thins notes out and can leave a headphone sounding dry or lifeless. This is why so many consumer headphones sound full and fun, leaning on a pronounced mid-bass and lower-mid, and why a genuinely neutral audiophile headphone can strike a listener as thin or dry by comparison even when it's the more accurate of the two.
Two distinctions matter. Note weight is not macro-dynamic weight: one is the thickness of every note, the other the heft of the big dynamic hits, and a headphone can have plenty of one and little of the other. And it isn't simply amount of bass, though they're related — it's about the body of notes across the whole range, not low-frequency quantity alone. A male vocal can have chest and a guitar can have wood without the bass being elevated at all.
There's a Goldilocks problem at the heart of it. Too much note weight and the sound turns cloying, congested, and slow, with the body of notes smearing into the spaces between them; too little and it's thin, dry, and unengaging. Lean tunings buy separation and speed at the cost of body; thick ones buy richness at the cost of clarity. The natural-sounding middle is the boring-but-hard target: enough body to sound human, not so much that everything wears a winter coat indoors.
Note weight is also one of the more preference-driven traits, which is why two listeners can disagree sharply about the same headphone — one hears rich and natural, the other thick and veiled. For a newcomer, the quickest way to hear it is a solo voice or piano: ask whether each note arrives with body and substance, or whether it sounds correct but slightly hollow, like a sketch of the real thing.