I am researching Digital Video cameras, techniques, gadgets, editing and I keep coming across a word that is never really explained by any of the books and articles that I read which contain it. The word is "Matte". It must have several meanings all having to do with the world of film and video making but I am seeing cross-eyed now. Is there a general defintion for the word Matte that I can plug in to understand each time I see it used.
Thanks for all the help!

What is a matte?

See http://www.indiana.edu/~telecom/faculty/krause/t354/week6.html

What is a matte box?

See http://dvcamerakit.com/HTML/Matte_Box_faqs.htm.