The Buyout is the transfer of rights to use a recording, usually in exchange for money (obviously). The exact usage is precisely defined regarding territory and duration or is granted without restriction (a so called "full buyout").
The counterpart to a buyout is the so-called "layout" or "Basic Session Fee" (BSF), which refers to the talent's rate for the studio time and production of the recording itself. However, the importance of BSFs is dwindling as more and more talents record in their own studios.