In chemistry, chemical symbol is the use of letters or symbols to represent chemical entities (atoms, molecules, or ions, etc.) using shorthand notation.  The basic example is the use of H to represent the element hydrogen.
In 1775, Torbern Bergman, in his 1775 A Dissertation on Elective Attractions, began to use of letters (a, b, ab, etc.) to represent chemicals in a generalized manner. 
“Chemical signs ought to be letters, for the greater facility of writing. I shall take therefore for the chemical sign, the initial letter of the Latin name of each elementary substance: but as several have the same initial letter, by writing the first two letters of the word.”— Jacob Berzelius (c.1810), Publication (Ѻ)