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If you are using an online bibliographic referencing tool, then select to create a reference of type "Dictionary" or "Encyclopedia" -- and put in the link.

Or in Wikipedia, use the "Cite this page" link in the left-hand column under "Tools" to have Wikipedia show you how to format a bibliographic entry for a page.

In your Works Cited or Bibliography, the Wikipedia entry should look like this in the alphabetical listing -- in MLA style:

Wikipedia contributors. "Butterfly Effect." Wikipedia, the Free Encyclopedia, Wikimedia Foundation, Inc, 2017, en.wikipedia.org/wiki/Butterfly_effect. Accessed 25 May 2017.

APA style would look like this:

Butterfly effect. (2017, May 25). In Wikipedia, The Free Encyclopedia. Retrieved May 25, 2017, from en.wikipedia.org/wiki/Butterfly_effect.​

And in the body of your paper, this information source can be referenced using an in-text citation like this -- in MLA style:

“In chaos theory, the butterfly effect is the sensitive dependence on initial conditions in which a small change in one state of a deterministic nonlinear system can result in large differences in a later state.” (Wikipedia contributors).

Or if there are several Wikipedia entries in your bibliography, then add enough information for the entry to be uniquely identified:

“In chaos theory, the butterfly effect is the sensitive dependence on initial conditions in which a small change in one state of a deterministic nonlinear system can result in large differences in a later state.” (Wikipedia contributors, "Butterfly effect").