in·stil·la·tion \ˌinztə̇ˈlāshən, ˌin(t)stə̇-\noun (-s) Etymology: Middle French & Latin; Middle French, from Latin instillation-, instillatio, from instillatus (past participle) + -ion-, -io -ion 1.: an act of instilling : introduction by instilling < repeated instillation of penicillin > 2.: something that is instilled or designed for instillation < an oily instillation > < silver instillation for use in the eyes of the newborn >