Upstream activator sequence (UAS): In a general sense, a UAS is a cis-acting regulatory element, usually located significantly upstream of the promoter and TATA box, to which transactivators bind to activate transcription of nearby genes. 

Although the term "UAS" can in principle be used to describe any cis-acting upstream sequence that activates transcription, in practice the term enhancer is used more often to describe these sequences in a general sense. UAS tends to be used to specifically describe the enhancer of the yeast $GAL1$ gene.