The Collaborative International Dictionary of English v.0.48:

Instillation \In`stil*la"tion\, n. [L. instillatio: cf. F.
   instillation.]
   The act of instilling; also, that which is instilled.
   --Johnson.
   [1913 Webster]

WordNet (r) 3.0 (2006):

instillation
    n 1: the introduction of a liquid (by pouring or injection) drop
         by drop [syn: instillation, instillment, instilment]
    2: a liquid that is instilled drop by drop