There has been much written here and elsewhere, about the development of hip-hop culture, in both its positive and negative aspects, from the teeny bopper article with LL Cool J shirtless to the academic article on the metaphorosis of diaspora identity. But there is not much talk on what rap technically is. Many people make far flung dissertations on what a hip-hop identity means, without looking into what the basic technical definition of rap is.
I think one of the reasons that this isn't discussed more is that the actual answer is somewhat overwhelming. Rap is a style of singing that accentuates rhythm, rather than melody. It should be noted that all singing uses some rhythm, just like all singing uses some melody. Rap just uses way more of the first than other types of music. What is interesting about the technical definition of rap is that there is nothing interesting about it. After listening and participating in rap for some ten years, there seems to be nothing to technically link the style with the culture that surrounds it, or that people believe surround it. There is nothing particularly angry, or black, or urban, or rebellious about the fact that singing emphasizes rhythm instead of melody.
What is more interesting about the appearance of rap is that many people responded to it as if it was a radical idea, as if rhythmic singing was something that a subculture of black youths invented out of nowhere during the 1970s. As I said above, all singing has some aspect of rhythm to it, and many forms of popular music, in rock and roll, country, or the Blues, had strong rhythmic aspects to their singing, long before the advent of hip-hop rapping. In fact, if you go back across the entire history and prehistory of music, there is a good chance that melodic singing was the rather late addition. Chanted music was probably shared around ancient campfires for thousands and tens of thousands of years before melody became the dominant force in music.
Which is not to say that the growth of rap technique wasn't gigantic during the Golden Age and Renaissance of Hip-Hop. It is just that it was just that that cultural tool was picked up by that particular culture as a means of expression. The art of rhythmic singing may end up being practiced by a quite different culture or sub-culture in the future.