Should I paraphrase or quotes definitions? In the literature review when I have many definitions from the literature can I paraphrase them or do I have to put them in quotes as they are?
If you use another source's definitions verbatim, then they must be placed in quotes; otherwise that's plagiarism. However, it should be pointed out that general facts and definitions—one that would be reasonably obvious to anyone who has, for instance, an undergraduate background in your field—need not be cited, so long as you are not literally quoting someone else. For instance, saying "Newton's second law states F = m × a" does not require a citation; saying


  The second law states that the net force on an object is equal to the rate of change (that is, the derivative) of its linear momentum p in an inertial reference frame [Wikipedia].


requires a citation to Wikipedia.