With the concept of tonic you might already know, if you can read this line, dedicated to fret.
The Greek word "tonos" and Latin tonus mean sound. From them and happened Italian was Tonica is a term which denotes the main, the sustainable level of the scale.
The tonic is the first, initial stage, therefore, its abbreviation, symbol - Roman numeral I or the letter T.
The same designation has the tonic triad, the chord built on the first degree major or minor modes.