Blocker - An issue (bug, feature, task) that blocks development and/or testing work, production could not run. An upcoming version that is affected by this issue cannot be released until it's addressed.
Critical - An upcoming version that is affected by this issue cannot be released until it's addressed. A critical bug is one that crashes the application, causes loss of data or severe memory leak.
Major - A request that should be considered seriously but is not a show stopper.
Minor - Minor loss of function, or other problem where easy workaround is present.
Optional - The request should be considered desirable but is not an immediate necessity.
Trivial - Cosmetic problem like misspelt words or misaligned text.
NOTE: Features that are intended to be advertised in marketing material should be tagged with Blocker or Critical priority.