User Activated Soft Fork
UASF is an acronym which stands for User Activated Soft Fork in the context of Bitcoin.
A UASF is the coordinated activation of a Bitcoin soft fork on a specified date and enforced by a supermajority of full nodes rather than relying on miners alone. In order to succeed, participating nodes must represent the so-called "economic majority" - users, exchanges and businesses with significant influence over the Bitcoin economy. A UASF requires developer, industry and user coordination. In the past, a UASF was successfully carried out to activate the P2SH soft fork (aka BIP16). On Feb 25, 2017, a pseudonymous contributor named Shaolin Fry proposed that UASF be used to activate Segregated Witness and later published details in BIP148.