contrary to popular belief, jazz is NOT playing any note with any chord. any note played with any chord can be explained. even if it is inexplicable, it just sounds good, which is what's really important.
the cool thing about jazz is that you learn all the rules and guidelines which apply to all the other genres...then you get to break them.
Jazz is an expression of thought and feeling. It can be slow and melancholy, or quick and light. It's simply what the musicians feel at the time. It's mainly instrumental, but sometimes there can be singing. I've read some of the definitions, apparently some dislike the lack of word. Well some things can't be expressed in words.
1. A music that originated in America, and credited to slaves who brought it over from West Africa. Has rhythmic foundations from the beats and syncopations from Africa. Has instrumental timbres from Spanish and European influences, as a result of the Spanish-American war and the diversity in New Orleans. Also has roots from ragtime piano, and eventually later on swing, blues, and bebop. Usually uses improvisation, but not always.
2. A verb similar to "swing". To give something a looser, more free-flowing, hipper feel. Can be done to a simple rhythm, or even things outside of music as in a "jazzed up place".
3. Often a thing that certain musical purists use to defend themselves as being the only decent kind of music left these days. Results in a kind of jazz elitism, where a jazz enthusiast believes he or she is better because he or she listens to the only kind of music that is still authentic and good.
2. This beat is too boring, we got to jazz it up a bit.
3. I don't listen to your repulsive rap, rock, or other mainstream pop. I'm close-minded and will forever be in my own little jazz bubble, which of course is the greatest gift America has for the world.