Assignment is the general term for the act of giving your rights to someone else. Without an assignment, or grant of rights, the use of your work by another entity would be an unlawful infringement of your copyrights.
As the original rights holder, you are the “Assignor” and the company acquiring the rights is the “Assignee.”
Those who will inherit the rights in the future may be referred to as heirs, successors or assigns.
