any axiom, law, or abstraction assumed and regarded as representing the highest possible degree of generalization.
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British Dictionary definitions for first principle
noun (usually plural)
one of the fundamental assumptions on which a particular theory or procedure is thought to be based
an axiom of a mathematical or scientific theory
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012