Principle vs Principal
Updated on 5/2/2013. Many thanks to Mike Ditto for pointing out that I had made a mistake in the original article and used the principle definition twice. I have updated the article with a better definition for principal.
I constantly see principle and principal used interchangeably. I see job posting for Principle Engineer positions as well as articles about Scheduling Principals in Java Applications (see http://www.javacodegeeks.com/2010/05/scheduling-principals-in-java.html). It is understandable that these words can be confused since they look and sound alike. They have very different meanings though.
When used as a noun, principal (according to http://dictionary.reference.com/browse/principal) refers to "a person who takes a leading part in any activity, as a play; chief actor or doer." As an adjective, it is "highest in rank or authority." This is the correct word to use when referring to Principal Engineer title or positions.
A principle is "a rule of action or conduct" (see http://dictionary.reference.com/browse/principle). It would have been the correct word to use in the title of the article referred to above.
The reality is that I will refuse to apply for a Principle Engineer position and I am very unlikely to read an article that misuses the word principals in a title. Call it a question of principles if you must.