Vexillology is the scientific study of flags and related emblems. It is concerned with research into flags of all kinds, both modern and historical, the creation of a body of practice for flag design and usage, and of a body of theory of flag development. Vexillology seeks to understand and explain the important part played by flags in the modern world.
The word is a synthesis of the Latin word vexillum, meaning "flag", and the suffix -logy, meaning "study of". The vexillum was a particular type of flag used by Roman legions during the classical era; its name is a diminutive form of the word vela meaning sail, and thus literally means "little sail". Unlike most modern flags, which are suspended from a pole or mast along a vertical side, the square vexillum was suspended from a horizontal crossbar along its top side, which was attached to a spear.
The term was coined in 1957 by the American scholar Whitney Smith, the author of many books and articles on the subject. It was originally considered a sub-discipline of heraldry, and is still occasionally seen as such. It is sometimes considered a branch of semiotics. It is formally defined in the FIAV (Fédération internationale des associations vexillologiques) constitution as "the creation and development of a body of knowledge about flags of all types, their forms and functions, and of scientific theories and principles based on that knowledge." A person who studies flags is a vexillologist; a person who designs flags is a vexillographer; and any person who simply likes, admires, or enjoys flags is a vexillolophile.