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nor- is a chemical acronym and prefix. It comes from the German nitrogen ohne Rest, "no residues at nitrogen." Residues are groups such as a methyl-group that are part of a larger molecule. Despite the word "nitrogen", it is also used in reference to other "stripped-down" molecules. For example, a norbornyl group (bicyclo[2.2.1]heptane) is a bornyl group (1,7,7-trimethylbicyclo[2.2.1]heptane) lacking the three different methyls at carbons.
Examples of chemicals with a nor- prefix include noradrenaline.