The Collaborative International Dictionary of English v.0.48:
Picket \Pick"et\, n. [F. piquet, properly dim. of pique spear,
pike. See Pike, and cf. Piquet.]
1. A stake sharpened or pointed, especially one used in
fortification and encampments, to mark bounds and angles;
or one used for tethering horses.
2. A pointed pale, used in marking fences.
3. [Probably so called from the picketing of the horses.]
(Mil.) A detached body of troops serving to guard an army
from surprise, and to oppose reconnoitering parties of the
enemy; -- called also outlying picket.
4. By extension, men appointed by a trades union, or other
labor organization, to intercept outsiders, and prevent
them from working for employers with whom the organization
is at variance. [Cant]
5. A military punishment, formerly resorted to, in which the
offender was forced to stand with one foot on a pointed
6. A game at cards. See Piquet.
Inlying picket (Mil.), a detachment of troops held in camp
or quarters, detailed to march if called upon.
Picket fence, a fence made of pickets. See def. 2, above.
Picket guard (Mil.), a guard of horse and foot, always in
readiness in case of alarm.
Picket line. (Mil.)
(a) A position held and guarded by small bodies of men
placed at intervals.
(b) A rope to which horses are secured when groomed.
Picketpin, an iron pin for picketing horses.