An order of battle was, in its original form during the European period of Medieval warfare, the order in which troops were positioned relative to the position of the Army commander. Today it refers to a listing of military units, often with equipment, location and other relevant information.
It was also applied to the disposition of ships in the line of battle during the age of sail. In the later transformation of its meaning during the European period of Early Modern warfare the order of battle came to mean the order in which the units manoeuvered or deployed onto the battlefield to form battle-lines, with the positioning on the right considered the place of greatest honour. This need to reflect the unit seniority led to the keeping of military staff records, in tabular form reflecting the compilation of units an army, their commanders, equipment, and locations on the battlefield.
During the Napoleonic wars the meaning of the order of battle changed yet again to reflect the changes in the composition of opposing forces during the battle owing to use of larger formations than in the previous century. With standardisation in organisation of field forces as part of regiments, brigades, divisions and Corps, the order of battle often became associated and confused with table of organisation which is a permanent composition of a given unit or formation according to army doctrine and to suit its staff administration operations. Napoleon also instituted the staff procedure of maintaining accurate information about the composition of the enemy order of battle, and tables of organisation, and this later evolved into an important function and an organisational tool used by military intelligence to analyse enemy capability for combat.
In its modern use the order of battle signifies the identification, command structure, strength, and disposition of personnel, equipment, and units of an armed force during field operations. Various abbreviations are in use, including OOB, O/B, or OB, while ORBAT remains the most common.