nor-

1. a combining form used in the names of chemical compounds which are the normal or parent forms of the compound denoted by the base words: l-norepinephrine.

combining form

1. indicating that a chemical compound is derived from a specified compound by removal of a group or groupsnoradrenaline
2. indicating that a chemical compound is a normal isomer of a specified compound

pref.

1. A precursor compound that differs from its successor by the absence of a radical group, usually methyl:norepinephrine.