Fusion just means you make a single DNA sequence by joining 2 or more other sequences. If one of the sequences is an enhancer, then the resulting joined sequence is an enhancer fusion. If one of the sequences is a promoter, then the resulting joined sequence is a promoter-fusion. You can, of course, get promoter-enhancer fusions, eg. "The process was analysed in transgenic flies carrying beta 1 enhancer-hsp70 promoter-lacZ fusion constructs." There isn't a strict distinction between promoters and enhancers. http://www.ncbi.nlm.nih.gov/books/NBK21745/#A2566