Ānantariya: Immediacy, is a name for that concentration of mind which is associated with the profound and shocking insight vipassanā that is present in any one of the 4 kinds of supra-mundane path consciousness see: ariya-puggala, and which therefore is the cause of the immediately following fruition phala consciousness. According to the Abhidhamma, this path-moment of the Stream-enterer sotapanna & the other Nobles is generated by the insight into the impermanence, misery and impersonality of all existence, reaching a certain threshold at that very moment and thus instantly transforming and ennobling the individual nature forever. It is mentioned under the name of ānantarika-samādhi in the Ratana Sutta Sn. v. 22 and in Pts.M. 1, Ñānakathā.
Maha Thera Nyanatiloka. Manual of Buddhist Terms and Doctrines, Buddhist Publication Society, first edition 1952.