Chemical Symbol
The one-, two- or three-letter abbreviation used internationally to represent an element.
Unit
Dimensionless
In depth
The first letter is always capitalised and any following letters are lower case, a convention that prevents ambiguity — CO is carbon monoxide while Co is cobalt. Many symbols derive from Latin or Greek names, and newly synthesised elements receive provisional three-letter symbols until IUPAC confirms a permanent name.
Examples in the real world
Fe from ferrum (iron), Na from natrium (sodium), Pb from plumbum (lead), and W from wolfram (tungsten).