The term 'buy-out' is used in cases where a client has hired a voice actor for a 'one price includes everything' type of contract. In short, if a voice actor 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, 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.