How to cite simple definitions when writing university papers?
Standards for papers and homework is not the same as for publications. Moreover, there is nothing fundamentally right or wrong about a definition as long as you introduce it as such ! (in University words mean what you decide they mean -- within reason, that is). You can definitely write something like


  In this paper I will use the definition of a virus as a '...' (1)


and refer to a footnote that says


  (1) definition taken from Wikipedia article Virus, retrieved on
  2014-10-04".


Of course if Wikipedia's definition is taken from a printed reference, use the latter.

If you want to be extra-cautious you can have a look at the change log and discussion of the Wikipedia article.