Definition
Plain language
A measure of how different two sets are based on what fraction of their items they share.
As stated in the literature
A set metric defined as one minus the size of the intersection over the size of the union, satisfying the metric axioms; used for fingerprint similarity in geometric memory substrates.
Also called: Jaccard
Why it matters: It's a simple, well-behaved way to compare sets — like document fingerprints or feature presences — without caring about counts or order.
For example, two recipes that share 4 ingredients out of a combined 10 have Jaccard distance 0.6.
Heard on the show
“… The overlap in which wrong answers appear is a median Jaccard of zero-point-seven-one — so about seventy percent shared — and the positions are clustered roughly …”Episode 140 — When a Reasoning Model Says "Let Me Double-Check" After It's Already Decided