Letter or combination of letters and numbers that represent various conditions or properties of an element, for example, a normal atom, O (oxygen); with its atomic weight, ¹⁶O; its atomic number, ₈¹⁶O; as a molecule, O₂; as an ion, O²⁺; in excited state, O*; or as an isotope, ¹⁸O.

1. A design used on a diagram to represent a component or to identify specific characteristics, quantities, or objects.

2. A sign letter or abbreviation used on a diagram or in an equation to represent a quantity or to identify an object.