Colonel (aka Col or COL) is a military rank of a commissioned officer, with corresponding ranks existing in almost every country in the world. It is also used in some police forces and other paramilitary rank structures. A colonel is typically in charge of a regiment in the army.
The term colonel derives from Latin columnella 'small column'. However, it was never actually a Roman rank. The system of ranks in the Roman military was quite different. As a rank the term arose in the late sixteenth century Italy where it referred to the officer in charge of a column (Italian colonna, plural colonne) or field force. The term is first attested as colonnello, but it is perhaps a truncation of something like capitano colonnello 'captain of the column, the captain designated to command the column'. In this context colonna seems to refer to a force marching in column, rather than to a battle formation — a battle or battlation of pike.
Today, a colonel is usually a military title rated as the highest, or the second-highest field rank below the general, or "flag" grades. In some small military forces, it can be the highest rank held.