What is the meaning of alienation clause?

alienation clause is a provision sometimes found in a promissory note or mortgage that provides that the balance of the secured debt becomes immediately due and payable at the option of the mortgagee upon the alienation of the property by the mortgagor. Alienation is usually broadly defined to include any transfer of ownership, title, interest, or estate in real property, including a sale by way of a contract for deed. Also called a due-on-sale clause. (See acceleration clause).