Colonel is a military rank, the top of the "field grade" system that divides officers into junior, field, and general/flag. In the NATO designation system (STANAG 2116), it is level OF-6, which is equivalent to the naval rank of captain, or national variants referring to "full" or "post" captain, or "ship captain." The next lower rank is commonly, but not exclusively, "lieutenant colonel". The next higher, again depending on the specific military organization, is "brigadier" or "brigadier general".
In modern militaries, typical command assignments at this level would be a regiment, brigade, or group. These are formations of 3,000 to 5,000 troops, or smaller specialized units that require a high degree of responsibility for field operations. While the trend has been to have the brigade as the basic field formation (see restructuring of the United States Army), some militaries have brigades made of more than one regiment. A colonel might also be assigned to a senior staff assignment, such as chief of staff for a division, or of a branch (e.g., intelligence) of a corps or higher command.
Promotion above colonel is substantially difficult in contemporary military organizations. Between 10 and 25 percent of U.S. colonels rise to brigadier general; the promotion from lieutenant colonel to colonel is also a major hurdle.
The derives from a Middle French modification of Old Italian colonnello (column of soldiers): colonel, diminutive of colonna column, from Latin columna . Historically, the colonel was the individual that raised a regiment-sized formation, and was senior among its officers. Such a regiment might be mercenaries, especially in the Middle Ages, or of volunteers, as in the American Civil War.
In some militaries, especially Asian communist, there are grades within colonel, distinct from colonel vs. lieutenant colonel. In China and North Korea, there are colonels and senior colonels.
Other militaries either do not have the title of brigadier/brigadier general; the Russian military, for example, goes from colonel to major general. Yet others consider brigadier (not brigadier general) to be the highest field rank, but not a general officer rank.
The Nazi SS ranks did not clearly map to more traditional military ranks; Standartenfuehrer was the commander of a regiment-equivalent, or "Standarte", but the next higher rank was Oberfuehrer, followed by Brigadefuehrer. Since the Oberfueher is between regimental and brigade command, some translate it as "senior colonel" and Brigadefuehrer as "brigadier general", where others translate Oberfuehrer as "brigadier general" and "Brigadefuehrer" as "major general".
"Colonel" is sometimes used as a modifier, as in "Colonel-General". Colonel-general is usually NATO grade OF-10, or "four star".
Many countries follow U.S. or British usages. In the U.S. military, which does have a brigadier general rank, colonels or equivalent wear a silver eagle.
The British and widespread Commonwealth usage has a crown and two diamonds. In this system, brigadier is not a general officer rank, so the brigadier wears a crown and three diamonds, where a major general wears crossed swords and a diamond. All general officer ranks are distinguished by crossed swords.
In the Russian system, a colonel (polkovnik) wears three small stars, much smaller than those of a general. There are other systems that use diamonds for staff grades, and reflect the Russian pattern of a colonel wearing three diamonds.
The term "colonel-in-chief" is honorary, used, for example, for the usually aristocratic patron of British regiments. "Usually" can cover a great deal, as, for example, with Colonel-in-Chief of the Norwegian Army, Sir Nils Olav, who is a penguin in the Edinburgh, Scotland zoo. 
- NATO codes for grades of military personnel: Agreed English texts, 1992, NATO Standardization Agreement (STANAG) 2116
- , Colonel, Merriam-Webster online
- Military penguin becomes a 'Sir', 15 August 2008