Software Bugs Severity 

Bug severity measures the impact a defect (or bug) can have on the development or functioning of an application feature when it is being used. 

Critical:

The bugs we need to fix immediately. These are the bugs that will prevent the product from working if it were released in that state. This includes crashing, freezing for which no workaround is possible.

High:

The same examples as with critical priority bugs. However high priority bugs don't interrupt the application/ workflow or at least can be avoided with a workaround.

Medium:

The bug does not cause a failure, does not impair usability and does not interfere with the fluent work of the system and programs.

Low:

The bug is aesthetic, is an enhancement or is a result of non-conformance to a standard - typing errors/ aesthetic inconsistencies

Usability:

Suggestions, features