An injective module is a type of module that has the property that any homomorphism from a submodule can be extended to the entire module. This means that if you have a module and a submodule, you can always find a way to map the submodule into the injective module without losing structure. Injective modules are crucial in understanding resolutions and decompositions of modules, as they play a key role in constructing split exact sequences and understanding projective modules.
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