The Collaborative International Dictionary of English v.0.48:

Antiphonal \An*tiph"o*nal\, a.
   Of or pertaining to antiphony, or alternate singing; sung
   alternately by a divided choir or opposite choirs. --Wheatly.
   -- An*tiph"o*nal*ly, adv.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Antiphonal \An*tiph"o*nal\, n.
   A book of antiphons or anthems.
   [1913 Webster]

WordNet (r) 3.0 (2006):

antiphonal
    adj 1: containing or using responses; alternating; "responsive
           reading"; "antiphonal laughter" [syn: responsive,
           antiphonal]
    2: relating to or resembling an antiphon or antiphony [syn:
       antiphonary, antiphonal]
    n 1: bound collection of antiphons [syn: antiphonary,
         antiphonal]