The term 'buy-out' is used in cases where a client has hired a voice over talent for a 'one price includes everything' type of contract. In short, if a voice over talent is hired at a 'buy-out' price, it means they will be paid once for the job, regardless of what is required, and no residuals.
Perhaps, one of the most famous 'buyouts' of all time was the usage of Vincent Price's voice for the 1982, Michael Jackson video, 'Thriller'.
The job was a 'buy-out' that paid Vincent Price $25,000. The video & song, in which his voice was used made over $250 million. Mr. Price made no residuals.