installation / instillation are similar-sounding terms with different meanings (referred to as homophones). To better understand the differences, see below for definitions, pronunciation guides, and example sentences using each term. 👇
installation: (noun) the act of installing something (as equipment).
instillation: (noun) the introduction of a liquid (by pouring or injection) drop by drop. (noun) a liquid that is instilled drop by drop.
installation: IH·n·s·t·ah·l·EY·sh·ah·n
instillation: NA
(Examples are Wikipedia snippets under the CC ShareAlike 3.0 license. Definitions drawn from WordNet.)