Can someone help me with this question about works cited?

I'm writing a compare and contrast essay for college about bulimia and anorexia. If my sentence was this: Bulimia is defined as: " an emotional disorder in which bouts of extreme overeating are followed nby depression and self-induced vomiting, purging, or fasting" I got that definition from dictionary.com....Do i say bulimia is defined by dictionary.com as...etc. Or do i cite it on my works cited? i'm sooo confused