A flag is a piece of fabric (most often rectangular) with distinctive colours and design. It is used as a symbol, a signalling device, or for decoration. The term flag is also used to refer to the graphic design employed, and flags have evolved into a general tool for rudimentary signalling and identification, especially in environments where communication is challenging (such as the maritime environment, where semaphore is used). Many flags fall into groups of similar designs called flag families. The study of flags is known as "vexillology" from the Latin vexillum, meaning "flag" or "banner".

National flags are patriotic symbols with widely varied interpretations that often include strong military associations because of their original and ongoing use for that purpose. Flags are also used in messaging, advertising, or for decorative purposes.

Some military units are called "flags" after their use of flags. A flag (Arabic: لواء) is equivalent to a brigade in Arab countries. In Spain, a flag (Spanish: bandera) is a battalion-equivalent in the Spanish Legion.

The origin of the flag is unknown and it remains unclear when the first flag was raised. Ships with vexilloids were represented on predynastic Egyptian pottery c. 3500 BC. In antiquity, field signs that can be categorised as vexilloid or "flag-like" were used in warfare, originating in ancient Egypt or Assyria. Examples include the Sassanid battle standard Derafsh Kaviani, and the standards of the Roman legions such as the eagle of Augustus Caesar's Xth legion and the dragon standard of the Sarmatians; the latter was allowed to fly freely in the wind, carried by a horseman, but depictions suggest that it bore more similarity to an elongated dragon kite than to a simple flag.

While the origin of the flag remains a mystery, the oldest flag discovered is made of bronze: a Derafsh or 'flag-like' Shahdad, which was found in Shahdad, Iran, and dates back to c. 2400 BC. It features a seated man and a kneeling woman facing each other, with a star in between. This iconography was found in other Iranian Bronze Age pieces of art.

Flags made of cloth were almost certainly the invention of the ancient peoples of the Indian subcontinent or the Zhou dynasty of Ancient China. Chinese flags had iconography such as a red bird, a white tiger, or a blue dragon, and royal flags were to be treated with a level of respect similar to that given to the ruler. Indian flags were often triangular and decorated with attachments such as a yak's tail and the state umbrella. Silk flags either spread to the Near East from China or it was just the silk itself, later fashioned by people who had independently conceptualized a rectangular cloth attached to a pole. Flags were probably transmitted to Europe via the Muslim world, where plainly coloured flags were used due to Islamic proscriptions. They are often mentioned in the early history of Islam and may have been copied from India.

In Europe, during the High Middle Ages, flags came to be used primarily as a heraldic device in battle, allowing easier identification of a knight over only the heraldic icon painted on the shield. Already during the high medieval period, and increasingly during the Late Middle Ages, city states and communes such as those of the Old Swiss Confederacy also began to use flags as field signs. Regimental flags for individual units became commonplace during the Early Modern period.