See tie-in in Webster's New World College Dictionary
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adjective
designating or of a sale in which two or more articles, one of which is usually scarce or in demand, are offered in combination, often at a reduced price, and cannot be bought separately
noun
a tie-in sale or advertisement
an article sold in this way
a connection or relationship
See tie-in in American Heritage Dictionary 4
noun
One thing that is related to or connected with another.