The Collaborative International Dictionary of English v.0.48:

Instillment \In*still"ment\, n.
   The act of instilling; also, that which is instilled.
   [Written also instilment.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

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