The present work deals with establishing of the exact meaning of the technical terms used by al-Kindi in the longest of his extant philosophical treatises, The Letter on the First Philosophy. On many occasions, however, when the meaning of a term appeared to be obscure in the Letter, the evidence of al-Kindī's usage of such a term has had to be brought forward from his other philosophical works in order to elucidate its meaning as accurately as possible.