Technical name for “many-to-many” mappings

A one-to-one mapping is a bijection. A many-to-one mapping is a "general function" and might also be a "surjective function".

Is there a similarly technical name for a many-to-many mapping, or should I just write "many-to-many" whenever describing such a thing. Same question for a "one-to-many" mapping (perhaps "inverse general function")?

EDIT: For what I mean by a "many-to-many" mapping:

Set 1: A, B, C, D
Set 2: W, X, Y, Z

A maps to W
B maps to W and X
C and D map to Y and Z (many-to-many relationship)

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What does many-to-many even mean? – Cameron Williams Jan 14 '14 at 3:08
In this case, multivalued is probably the right term, as janmarqz suggests. – Cameron Williams Jan 14 '14 at 3:13