You can think of buoyancy as the push that water gives back. Hold a ball under the surface and you can feel the water working against your hand; let go, and the ball comes straight up. Every use of the word keeps two parts of that picture: there is something pressing down, and there is a steadier force answering it from below. Which part the word points at depends on the sentence — sometimes the push itself, sometimes the object that the push keeps up, and sometimes a person or a market that goes under for a while and then rises again.
From the word’s history
Wiktionary derives buoyancy from buoyant plus -cy, or from buoy plus -ancy. A buoy is the marker that sits on the surface and will not stay pushed under; the suffix turns that behaviour into something you can name and measure.
Wiktionary 記 buoyancy 來自 buoyant（會浮的）加 -cy，或 buoy（浮標）加 -ancy。浮標就是那個永遠壓不下去、總是留在水面上的東西。字尾 -cy／-ancy 把這種行為變成一個名詞，於是「會浮」這件事就成了可以談、可以量的性質。
Wiktionary entry for buoyancy (kaikki extract, source_ref kaikki:3f6f69a190036ae4:926980649): From buoyant + -cy or buoy + -ancy.
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