Letter names for musical pitches in this online search tool use the system known loosely as scientific pitch notation (or American Standard Pitch Notation).
It is often abbreviated “ASA”, referring to the Acoustical Society of America. This method, given below, is simple and easy to remember: just think that “middle C” is C4, and the complete octave below it is C3 – B3.
On the piano, C8 would be the top note, and C1 would be almost the lowest note. (The next lower octave wold be C0 – B0.)