Source : Webster's Revised Unabridged Dictionary (1913)

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

Source : WordNet®

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