Complex sentence: A reservoir (etymology: from French "réservoir" a "storehouse") is an enlarged natural or artificial lake, storage pond or impoundment created using a dam or lock to store water.
Split into simpler sentences: A reservoir (etymology: from French "réservoir" a "storehouse") is an storage space for fluids. The reservoir usually mean an enlarged natural or artificial lake, storage pond or impoundment created using a dam or lock to store water.