Freestyle rap commonly refers to rap lyrics which are improvised through a cappella or with instrumental beats.
It is similar to other improvisational music such as jazz. A rap group from Los Angeles describes it as being "like a jazz solo". Rap battles are sometimes improvised in this way.
In the book How to Rap, Big Daddy Kane and one member of Freestyle Fellowship note that originally a freestyle was a written rap on no particular subject – Big Daddy Kane says, “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.”