Normally, the complexity of a defect
The severity of a defect, in the context of testing, refers to the effect the defect has on the software product/system. So for each defect, we need to identify like how critical the defect is, and how much effect it is going to have on our product. A defect that completely hampers or blocks testing of the product can be considered as a critical defect. When a defect has only minor or no effect on the final result of the product, we can consider it as a low defect.
(Pic source: http://www.softwaretestinghelp.com/how-to-set-defect-priority-and-severity-with-defect-triage-process/)