Definition of "one-to-many"

(mathematics, of a function) Having the property that the same argument may yield multiple values, but different arguments never yield the same value.  examples

(mathematics, logic, of a relationship between two sets) having the property that an element of one set may be assigned by the relationship to several elements in the other set, but that a given element in the second set can have only one member of the first set assigned to it.  examples

From a single source to multiple recipients.  examples