- (metrology) Symbol for the attoampere, an SI unit of electrical current equal to 10−18 amperes.
- (obsolete, metrology) Symbol for the abampere, a CGS unit of electrical current equal to 10 amperes.
Usage notes 
The use of the abampere and related CGS units had largely been displaced by the MKS system by 1964 when the SI prefix atto- came into use. Thus a knowledge of when the quantity in question was written will usually be sufficient to determine which meaning of aA was intended.