In physics, the ampere (symbol: A, often informally abbreviated to amp) is the SI base unit used to measure electrical currentss. By definition, 1 ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 metre apart in vacuum, would produce between these conductors a force equal to 2x10^-7 newton per meter of length.

The ampere is named after André-Marie Ampère, one of the main discoverers of electromagnetism.

The unit of electric charge, the coulomb, is defined in terms of the ampere: 1 coulomb is the amount of electric charge carried in a current of 1 ampere flowing for 1 second.

Due to the difficulty in measuring the force between two conductors, the so called "international ampere" or "statampere" was proposed, defined in terms of deposition rate of silver. It is equal to 0.99985 amperes. This alternate unit is now considered obsolete.