Colonel (/ˈkɜːrnəl/ ^ⓘ KUR-nəl; abbreviated as Col., Col, or COL) is a senior military officer rank used in many countries. It is also used in some police forces and paramilitary organizations.

In the 17th, 18th, and 19th centuries, a colonel was typically in charge of a regiment in an army. Modern usage varies greatly, and in some cases, the term is used as an honorific title that may have no direct military relationship. In some smaller military forces, such as those of Monaco or the Vatican, colonel is the highest rank.

Equivalent naval ranks may be called captain or ship-of-the-line captain. In the Commonwealth's air force ranking system, the equivalent rank is group captain.

By the end of the late medieval period, a group of "companies" was referred to as a "column" of an army. According to Raymond Oliver, c. 1500, the Spanish began explicitly reorganizing part of their army into 20 colunelas or columns of approximately 1,000–1,250 soldiers. Each colunela was commanded by a cabo de colunela or column head. Because they were crown units who are directly under the control of the monarch or sovereign of a country, the units were also confusingly called coronelas, and their commanders coronels. Evidence of this can be seen when Gonzalo Fernández de Córdoba, nicknamed "the Great Captain", divided his armies in coronelías, each led by a coronel, in 1508.

Later, in the 16th century, the French army adopted this organizational structure, renaming colunelas regiments. Even so, they simply Gallicized colunela to the French colonel and pronounced it as written. The English then copied the unit and rank from the French. However, for reasons unknown, the English adopted the Spanish pronunciation of coronel, and after several decades of use shortened it to its current two-syllable pronunciation "kernel". Colonel is linked to the word column (from Latin: columna; Italian: colonna; French: colonne) in a similar way that brigadier is linked to brigade, although in English this relationship is not immediately obvious.

With the shift from primarily mercenary to primarily national armies in the course of the 17th century, a colonel (normally a member of the aristocracy) became a holder (German Inhaber) or proprietor of a military contract with a sovereign. The colonel purchased the regimental contract—the right to hold the regiment—from the previous holder of that right or directly from the sovereign when a new regiment was formed or an incumbent was killed.

As the office of colonel became an established practice, the colonel became the senior captain in a group of companies that were all sworn to observe his personal authority—to be ruled or regimented by him. This regiment, or governance, was to some extent embodied in a contract and set of written rules, also referred to as the colonel's regiment or standing regulation(s). By extension, the group of companies subject to a colonel's regiment (in the foregoing sense) came to be referred to as his regiment (in the modern sense) as well.

In French usage of this period, the senior colonel in the army or, in a field force, the senior military contractor, was the colonel general, and, in the absence of the sovereign or his designate, the colonel general might serve as the commander of a force. The position, however, was primarily contractual and it became progressively more of a functionless sinecure. The head of a single regiment or demi-brigade would be called a 'mestre de camp' or, after the Revolution, a 'chef de brigade'.