any of a family (Paramyxoviridae) of RNA viruses, including those causing mumps, measles, and parainfluenza
See paramyxovirus in American Heritage Dictionary 4
nounpl.par·a·myx·o·vi·rus·es
Any of a group of viruses that contain RNA and are similar to but larger and more variable in size than the related myxovirus. The paramyxoviruses include the Sendai virus, the parainfluenza viruses, and the viruses that cause measles and mumps.