Piece of cloth used as an emblem or symbol for nationalistic, religious, or military displays, or as a means of signaling. Flags have been used since ancient times. Many localities and public bodies, as well as shipping lines, schools, and yacht clubs, have their own distinguishing flags.
symbolism
The Stars and Stripes, also called Old Glory, is the flag of the US; the 50 stars on a field of blue represent the 50 states now in the Union, and the 13 red and white stripes represent the 13 original colonies. Each state also has its own flag. The US presidential standard displays the American eagle, surrounded by 50 stars.
The British national flag, the Union Jack, unites the crosses of St George, St Andrew, and St Patrick, representing England, Scotland, and Ireland. The flags of Australia and New Zealand both incorporate the Union Jack, together with symbols of the Southern Cross constellation.
The flags of the Scandinavian countries bear crosses; the Danish Dannebrog (“strength of Denmark”) is the oldest national flag, used for 700 years. The Swiss flag inspired the Red Cross flag with colors reversed. Muslim states often incorporate in their flags the crescent emblem of Islam and the color green, also associated with their faith. Similarly Israel uses the Star of David and the color blue.
The flag of the former USSR placed the crossed hammer and sickle, which represented the workers of town and country, on a red field, the emblem of revolution.
The Canadian flag has a maple-leaf design and that of Japan a red disc symbolizing the sun.
signals
A flag is flown upside down to indicate distress; is dipped as a salute; and is flown at half-mast to show mourning. The “Blue Peter”, blue with a white center, announces that a vessel is about to sail; a flag half red and half white, that a pilot is on board. A yellow flag means “plague”.
1. An emblem flown as a symbol of nationality; SYN. national flag, ensign.
2. A rectangular piece of fabric used as a signalling device; SYN. signal flag.
3. Usually rectangular piece of cloth of distinctive design.
4. Stratified stone that splits into pieces suitable for paving; SYN. flagstone.
5. A conspicuously marked or shaped tail.
Any of various types of indicators used for identification of a condition or event; for example, a character that signals the termination of a transmission.

ETYM From Flag an ensign.
1. To communicate or signal with a flag.
2. To decorate with flags, as of buildings.
3. To provide with a flag.

In botany, another name for iris, especially yellow flag Iris pseudacorus, which grows wild in damp places throughout Europe; it is a true water plant but adapts to border conditions. It has a thick rhizome, stiff, bladelike, monocotyledonous leaves, and stems up to 150 cm/5 ft high. The flowers are large and yellow.

Broadly, a marker of some type used by a computer in processing or interpreting information; a signal indicating the existence or status of a particular condition. Flags are used in such areas as communications, programming, and information processing. Depending on its use, a flag can be a code, embedded in data, that identifies some condition, or it can be one or more bits set internally by hardware or software to indicate an event of some type, such as an error or the result of comparing two values.
2. In the HDLC communications protocol, a flag is the unique series of bits 01111110, used to start and end a transmission frame (message unit). See also HDLC.
In computing, an indicator that can be set or unset in order to signal whether a particular condition is true—for example, whether the end of a file has been reached, or whether an overflow error has occurred. The indicator usually takes the form of a single binary digit, or bit (either 0 or 1).