Elementary charge
The elementary charge, usually denoted by e, is a fundamental physical constant, defined as the electric charge carried by a single proton (+1 e) or, equivalently, the negative of the electric charge carried by a single electron, which has charge −1 e. In SI units, the coulomb is defined such that the value of the elementary charge is exactly e = 1.602176634×10−19 C. Since the 2019 revision of the SI, the seven SI base units are defined in terms of seven fundamental physical constants, of which the elementary charge is one.