I was reading a scientific recommendation the other day when I came upon the phrase "principle investigator." It had me worried. Did the project require someone to investigate our principles? Were we guilty of ethical lapses? Were our rules in need of scrutiny?
I hope the answer to those questions is no. If it is no, then the correct phrase is "principal investigator." It means "chief investigator" or "lead investigator."
If you'd like to read more about the correct use of principal and principle, read my posts The Principal Principle and Business Cards & Principals.