Logical constant

In logic, a logical constant of a language formula_1 is a symbol that has the same semantic value under every interpretation of formula_1. Two important types of logical constants are logical connectives and quantifiers. The equality predicate (usually written '=') is also treated as a logical constant in many systems of logic.
One of the fundamental questions in the philosophy of logic is "What is a logical constant?"; that is, what special feature of certain constants makes them "logical" in nature?
Some symbols that are commonly treated as logical constants are:
Many of these logical constants are sometimes denoted by alternate symbols ("e.g.", the use of the symbol "&" rather than "∧" to denote the logical and). Defining logical constants is a major part of the work of Gottlob Frege and Bertrand Russell.