The Collaborative International Dictionary of English v.0.48:

Distention \Dis*ten"tion\, n. [L. distentio: cf. F. distension.]
   1. The act of distending; the act of stretching in breadth or
      in all directions; the state of being Distended; as, the
      distention of the lungs.
      [1913 Webster]

   2. Breadth; extent or space occupied by the thing distended.
      [1913 Webster]

WordNet (r) 3.0 (2006):

distention
    n 1: the state of being stretched beyond normal dimensions [syn:
         dilatation, distension, distention]
    2: the act of expanding by pressure from within [syn:
       distention, distension]