Brigade is a term from military science which refers to a group of several battalions (typically two to four), and directly attached supporting units (normally including at least an artillery battery and additional logistic support).
A brigade is smaller than a division and roughly equal to or a little larger than a regiment. Strength typically ranges between 1,500 and 3,500 personnel.
In the United States Army the term brigade is used instead of the term regiment, except in the cavalry. This is because a regiment has a fixed structure, whereas a brigade can be changed to suit the mission's needs.
In the United State Marine Corps , brigades are only formed for certain missions. The Marines have intact regimental structures unlike the army. A Marine brigade is formed only for special expeditionary duty where it is outfitted like a smaller MEF. For example, TF TARAWA (2d MEB) during the OIF campaign.
In the British Army, the brigade has been the smallest tactical formation for more than two centuries, since regiments are either administrative groupings of battalions (in the infantry) or battalion-sized units (in the cavalry).
A brigade is usually commanded by a brigadier general, brigadier or colonel. In colonial powers such as the British Empire, brigades frequently garrisoned isolated colonial posts, and their commanders had substantial discretion and local authority.
The term derives from Italian "brigata" or Old French "brigare," for company, which in turn derives from a Celtic root "briga," which means strife.