In some physics textbooks and articles, certain quantities are defined with the opposite sign from that which is used in other publications. This lack of standardization is a frequent source of confusion, misunderstandings, and even outright errors. A standard choice of sign for a definition is called a sign convention. Sometimes, this term is used more broadly to include factors of i and 2π.
A noncomprehensive list of sign conventions:
It is often considered good form to state explicitly which sign convention is to be used at the beginning of each book or article.