#1 phosphorus n. [P, atomic number 15] a multivalent nonmetallic element of the nitrogen family that occurs commonly in inorganic phosphate rocks and as organic phosphates in all living cells; is highly reactive and occurs in several allotropic forms. wn
#2 phosphorus n. [morning star, daystar, Phosphorus, Lucifer] a planet (usually Venus) seen just before sunrise in the eastern sky. wn
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