DENISON:

Eng lish: patronymic from the personal name "Dennis."  Possibly an Americanized form of cognates in other languages, for example the Russian "Denisov," from Denis, or Ukrainian "Denysevich", from Denys.

DENIS: French, Spanish (Denis) and Portuguese: from the personal name Denis, a variant of Dennis.  Ukrainian: from the personal name "Denys."  [see Dennis]

From: Dictionary of American Family Names, Oxford Univ Press, ISBN 0-19-508137-4. Courtesy of Ancestry.com's search tool.