Flocculation describes how readily yeast cells clump together and settle out of suspension once fermentation slows down — a property that varies genuinely by strain, and one of the practical characteristics (alongside attenuation and temperature range) worth checking before picking a yeast for a given recipe.
Why this matters beyond just how clear the beer looks
High-flocculation strains drop out of suspension quickly, producing a beer that clears fast without much intervention and, because the yeast stops actively fermenting sooner as it settles, can sometimes leave slightly more residual sweetness behind at the margin. Low-flocculation strains stay in suspension considerably longer, which is exactly the trait modern hazy IPA yeast selection deliberately leans on — a genuinely hazy appearance depends partly on yeast that simply refuses to settle quickly, not just on protein-rich grain.
A trait to check alongside attenuation, not instead of it
Flocculation and attenuation are separate, independent properties — a strain can flocculate quickly while still attenuating highly, or clear slowly while finishing only moderately dry — so checking both figures on a strain's data sheet, rather than assuming one predicts the other, avoids picking a yeast that surprises you on either clarity or finishing gravity when only one number was actually checked.