[DanaC]

From wikipedia:

Original definition[edit]
In the book How to Rap, Big Daddy Kane and Myka 9 note that originally a freestyle was a spit on no particular subject – Big Daddy Kane said, "in the ’80s when we said we wrote a freestyle rap, that meant that it was a rhyme that you wrote that was free of style... it's basically a rhyme just bragging about yourself."

[7] Myka 9 adds, "back in the day freestyle was bust[ing] a rhyme about any random thing, and it was not a written rhyme or something memorized".[6]

Divine Styler says: "in the school I come from, freestyling was a non-conceptual written rhyme... and now they call freestyling off the top of the head, so the era I come from it's a lot different".[8] Kool Moe Dee also refers to this earlier definition in his book, There's A God On The Mic:[9]
There are two types of freestyle. There's an old-school freestyle that's basically rhymes that you've written that may not have anything to do with any subject or that goes all over the place. Then there's freestyle where you come off the top of the head.[10]

In old school hip-hop, Kool Moe Dee claimed that improvisational rapping was instead called "coming off the top of the head",[11] and Big Daddy Kane stated, "off-the-top-of-the-head [rapping], we just called that "off the dome" — when you don't write it and [you] say whatever comes to mind".[7]

Referring to this earlier definition (a written rhyme on non-specific subject matter) Big Daddy Kane stated, "that's really what a freestyle is"[7] and Kool Moe Dee refers to it as "true"[12] freestyle, and "the real old-school freestyle".[13] Kool Moe Dee suggests that Kool G Rap's track 'Men At Work' is an "excellent example"[12] of "true"[12] freestyle, along with Rakim's "Lyrics of Fury".[14]

https://en.wikipedia.org/wiki/Freestyle_rap