Several students have asked how to reference this and here is what we would suggest.
Firstly look at the source you have read the definition in, this could be a book, journal or a reputable website.
Then use this source in your reference list. For example:
World Health Organization. (n.d.). Frequently asked questions. Retrieved from http://www.who.int/suggestions/faq/en/
In the text you would then cite the source you read the definition in:
In 1946 the World Health Organization (WHO) defined health as “a state of complete physical, mental and social well-being and not merely the absence of disease or infirmity” (WHO, n.d., para. 1).
NB: If you use a direct quote you must use speech marks and include a page or paragraph number.