Threshold Signature
A signature scheme in which a (t, n) threshold of signers must collaborate to produce a single signature that verifies under one shared public key. BLS signatures (Boneh, Lynn & Shacham 2001) are the most widely deployed instance: signature aggregation is non-interactive (sum of group elements) and verification cost is roughly that of a single signature. The cryptographic primitive that makes HotStuff’s linear communication complexity possible: 2f+1 votes aggregate into a single constant-size Quorum Certificate.