The Collaborative International Dictionary of English v.0.48:

Affusion \Af*fu"sion\ ([a^]f*f[=u]"zh[u^]n), n. [Cf. F.
   affusion.]
   The act of pouring upon, or sprinkling with a liquid, as
   water upon a child in baptism. Specifically: (Med) The act of
   pouring water or other fluid on the whole or a part of the
   body, as a remedy in disease. --Dunglison. [archaic] --AS
   [1913 Webster]

WordNet (r) 3.0 (2006):

affusion
    n 1: the act of baptizing someone by pouring water on their head

Moby Thesaurus II by Grady Ward, 1.0:

40 Moby Thesaurus words for "affusion":
   aspergation, aspersion, baptism, baptismal gown,
   baptismal regeneration, baptistery, baptizement, bath, bathing,
   bedewing, chrismal, christening, dampening, damping, deluge,
   dewing, drowning, flooding, font, hosing, hosing down,
   humidification, immersion, infusion, inundation, irrigation,
   laving, moistening, rinsing, sparging, spattering, splashing,
   splattering, spraying, sprinkling, submersion, swashing,
   total immersion, watering, wetting