Source : Webster's Revised Unabridged Dictionary (1913)

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

Source : WordNet®

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