It is a word of Latin origin. It means the same thing, equal or equal. You can do the function of demonstrative pronoun or adverb in our language . It can also mean or not . It is used to avoid word repetition.
'Idem' is a pronoun and adverb that is used as "the same". It is a Castilianization of the Latin idem, with equal pronunciation and meaning. Its abbreviation is id. .