UASF(User Activated Soft-Fork)
UASF refers to the activation of a soft-fork without having obtained support from a large minority of miners. Normally, soft-forks are activated if they receive enough support from miners (95%), measured by counting the number of blocks signaling for a soft-fork proposal among the last 2016 mined blocks (as per BIP 9). UASF usually refers to BIP 148, a proposed mechanism to force the activation of the Segregated Witness (SegWit) BIP. With this UASF, nodes implementing it would start rejecting any block not signaling support for SegWit between August 1st 2017 to November 15th 2017. The idea is that if a sufficient number of validating node starts rejecting blocks not supporting SegWit, miners would be incentivized to support it as not to get the block they mine orphaned, and thus lose their reward.