Welcome as a new member to this forum. However please read the FAQS that were sent to you on registration. One of them is:
"1) Please execute a search regarding the potential topic you are considering posting (located in the upper right corner). There are hundreds of threads on various topics that may answer your question existing in the archives. Topics posted which already exist in the archives will be removed without explanation."
On such basics a definitions you should also do a search on Google or some other search engine.
I did a search on MSN using "Functional Testing definition" and the first result displayed was: