What is Lex?

Legal Definition
The law. A law for the government of mankind in society. Among the ancient Romans, this word was frequently used as synonymous with right, jus. When put absolutely, lex meant the Law of the Twelve Tables.
-- Bouviers Law Dictionary
Legal Definition
Lat. In the Roman law. Law; a law; the law. This term was often used as the synonym of jus, in the sense of a rule of civil conduct authoritatively prescribed for the government of the actions of the members of an organized jural seciety. In a more limited and particular sense, it was a resolution adopted by the whole Roman "populus" (patricians and plebians) in the comitia, on the motion of a magistrate of senatorial rank, as a consul, a praetor, or a dictator. Such a statute frequently took the name of the proposer; as the lex Falcidia, lex Cornelia, etc.
See also
-- Black's Law Dictionary
Legal Definition
-- Ballentine's Law Dictionary