Can we have a citation in the Works Cited and not use it in an essay? I have a citation of an online article that I used in my research for an essay, yet I haven't referenced this citation anywhere in the essay. Should I keep the citation, or delete it because nothing refers to it?
A Works Cited by definition includes only the...works that you cited. So any extraneous entries should be removed.

In contrast, a bibliography can include all related work, even if you don't specifically refer to it. The goal of a bibliography is not only to cite the author's sources, but also to provide recommended reading for the reader. 

In scientific journals, my experience is with the former. If you think a particular article has value, you can refer to it in the text, even obliquely (e.g., "X is widely discussed in the literature [1-15]. In this work, ...")