The Ampere:

The ampere, represented by the uppercase non-italicized English letter A (or shortened as amp), is the unit of electric current. A flow of around 6.241506 x 10¹⁸ electrons per second past a given fixed point in an electrical conductor generates an electrical current of 1 A.

Different units smaller than the ampere are frequently employed to measure or define current. A milliampere (mA) is one-thousandth of an ampere, or a flow of 6.241506 x 10¹⁵ electrons per second past a given fixed point. A microampere (πA) is one-millionth or 10^-6 of an ampere, or a flow of 6.241506 x 10¹² electrons per second. The nanoampere (nA) is 10^-9 of an ampere; it is the smallest unit of electric current you are probable to hear about or use. It symbolizes a flow of 6.241506 x 10⁹ electrons per second past a given fixed point.

The official definition of the ampere is highly hypothetical: 1A is the quantity of constant charge-carrier flow via two straight, parallel, infinitely thin, absolutely conducting media placed 1m distant in a vacuum which results in a force between the conductors of 2 x 10^-7 newton per linear meter. There are two major problems with this definition. At first, we haven't defined the word newton yet; secondly, this definition requests you to visualize some hypothetically ideal objects which cannot exist in the real world. However, there you have it: the physicist venturing into the mathematician's back yard again. It has been thought that mathematicians and physicists cannot live with each other and they cannot live without each other.