Scientific pitch notation

Scientific pitch notation (SPN), also known as American standard pitch notation (ASPN) and international pitch notation (IPN), is a method of specifying musical pitch by combining a musical note name (with accidental if needed) and a number identifying the pitch's octave. Although scientific pitch notation was originally designed as a companion to scientific pitch (see below), the two are not synonymous.

Source: Wikipedia — Scientific pitch notation (CC BY-SA 4.0)