The Collaborative International Dictionary of English v.0.48:

Irrigation \Ir`ri*ga"tion\, n. [L. irrigatio: cf. F.
   irrigation.]
   The act or process of irrigating, or the state of being
   irrigated; especially, the operation of causing water to flow
   over lands, for nourishing plants.
   [1913 Webster]

WordNet (r) 3.0 (2006):

irrigation
    n 1: supplying dry land with water by means of ditches etc
    2: (medicine) cleaning a wound or body organ by flushing or
       washing out with water or a medicated solution