Achath (Heb.). The one, the first, feminine; achad being masculine. A Talmudic word applied to Jehovah. It is worthy of note that the Sanskrit term ak means one, ekata being “unity”, Brahmâ being called ák, or eka, the one, the first, whence the Hebrew word and application.
Source: H.P.Blavatsky - The Theosophical Glossary