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

Moby Thesaurus II by Grady Ward, 1.0:

63 Moby Thesaurus words for "irrigation":
   ablution, affusion, aspergation, aspersion, baptism, bath, bathing,
   bedewing, cleaning out, dampening, damping, deluge, dewing, douche,
   douching, drowning, elution, elutriation, enema, flooding, flush,
   flushing, flushing out, hosing, hosing down, humidification,
   immersion, inundation, lathering, lavabo, lavage, lavation, laving,
   moistening, mopping, mopping up, rinse, rinsing, scouring, scrub,
   scrubbing, scrubbing up, shampoo, soaping, sparging, spattering,
   splashing, splattering, sponge, sponging, spraying, sprinkling,
   submersion, swabbing, swashing, wash, washing, washing up, washout,
   washup, watering, wetting, wiping up



Easton's 1897 Bible Dictionary:

Irrigation
   As streams were few in Palestine, water was generally stored up
   in winter in reservoirs, and distributed through gardens in
   numerous rills, which could easily be turned or diverted by the
   foot (Deut. 11:10).

     For purposes of irrigation, water was raised from streams or
   pools by water-wheels, or by a shaduf, commonly used on the
   banks of the Nile to the present day.


Bouvier's Law Dictionary, Revised 6th Ed (1856):

IRRIGATION. The act of wetting or moistening the ground by artificial means.
     2. The owner of land over which there is a current stream, is, as such,
the proprietor of the current. 4 Mason's R. 400. It seems the riparian
proprietor may avail himself of the river for irrigation, provided the river
be not thereby materially lessened, and the water absorbed be imperceptible
or trifling. Ang. W. C. 34; and vide 1 Root's R. 535; 8 Greenl. R. 266; 2
Conn. R. 584; 2 Swift's Syst. 87; 7 Mass. R. 136; 13 Mass. R. 420; 1 Swift's
Dig. 111; 5 Pick. R. 175; 9 Pick. 59; 6 Bing. R. 379; 5 Esp. R. 56; 2 Conn.
R. 584; Ham. N. P. 199; 2 Chit. Bl. Com. 403, n. 7; 22 Vin. Ab. 525; 1 Vin.
Ab. 657; Bac. Ab. Action on the case, F. The French law coincides with our
own. 1 Lois des Batimens, sect. 1, art. 3, page 21.