pertaining to or designating a sale in which the buyer in order to get the item desired must also purchase one or more other, usually undesired, items.

of or pertaining to two or more products advertised, marketed, or sold together.

an arrangement or campaign whereby related products are promoted, marketed, or sold together

a book and movie tie-in.

a tie-in sale or advertisement.

an item in a tie-in sale or advertisement.

any direct or indirect link, relationship, or connection

There is a tie-in between smoking and cancer.

noun

connection , correlation , interconnection , interdependence , interrelationship , link , linkage , relationship