A picket (archaically, picquet [variant form piquet]) is a soldier, or small unit of soldiers, placed on a line forward of a position to provide warning of an enemy advance. It can also refer to any unit (for example, an aircraft or ship) performing a similar function.
Picket (Fr. piquet, a pointed stake or peg, from piquer, to point or pierce), is thought to have originated in the French army about 1690, from the circumstance that an infantry company on outpost duty dispersed its musketeers to watch, the small group of pikemen called piquet remaining in reserve. It was in use in the British Army before 1735 and probably much earlier.
Picket now refers to a soldier or small unit of soldiers maintaining a watch. This may mean a watch for the enemy, or other types of watch e.g. "fire picket". This can be likened to the art of sentry keeping.
A staggered picket consists of, for example, two soldiers where one soldier is relieved at a time. This is so that on any given picket one soldier is fresh, having just started the picket, while the other is ready to be relieved. Although each soldier is required to maintain watch for the full duration of a shift, halfway through each shift a new soldier is put on watch.