Asat (Sk.). A philosophical term meaning “non-being”, or rather non-be-ness. The “incomprehensible nothingness”. Sat, the immutable, eternal, ever-present, and the one real “Be-ness” (not Being) is spoken of as being “ Born of Asat, and Asat begotten by Sat”. The unreal, or Prakriti, objective nature regarded as an illusion. Nature, or the illusive shadow of its one true essence.
Source: H.P.Blavatsky - The Theosophical Glossary