Latin word once used in various phrases in English, often in legal language, where it means "the condition of something, the matter in hand or point at issue;" literally "thing" (see re). For example res ipsa loquitur "the thing speaks for itself;" res judicata "a point decided by competent authority."

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Definitions of res from WordNet

res (n.)
a widely distributed system consisting of all the cells able to ingest bacteria or colloidal particles etc, except for certain white blood cells;
Synonyms: reticuloendothelial system