Dictionary.com's 21st Century Lex defines spyware as:
"any software that covertly gathers information about a user while he/she navigates the Internet and transmits the information to an individual or company that uses it for marketing or other purposes."
Spyware is by far the most difficult term to define due to the fact that there are so many variants with so many different names.
For the purpose of this site, we refer to everything that is not a traditional virus as spyware.