Viruses are small programs that can attach themselves to other programs and are capable of reproducing and spreading from program to program. Anything that fits that definition qualifies as a virus. The virus may do other things when it attaches itself to a program, or it may simply spread. Most Mac viruses do not do much more than spread.
Another type of program often confused with a virus is called a "Trojan Horse". A Trojan Horse will appear to be a normal program, but will actually do damage. It does not reproduce itself, or attach itself to other programs.