The Collaborative International Dictionary of English v.0.48:

Laxation \Lax*a"tion\, n. [L. laxatio, fr. laxare to loosen, fr.
   laxus loose, slack.]
   The act of loosening or slackening, or the state of being
   loosened or slackened.
   [1913 Webster]

WordNet (r) 3.0 (2006):

laxation
    n 1: the elimination of fecal waste through the anus [syn:
         defecation, laxation, shitting]
    2: the act of making something less tight [syn: loosening,
       laxation] [ant: tightening]