What does "picket" mean?

Additional senses

1. 2.Etym: [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.
2. 3.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]
3. 4.A military punishment, formerly resorted to, in which the offender was forced to stand with one foot on a pointed stake.
4. 5.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.
5. 6.To fortify with pointed stakes.
6. 7.To inclose or fence with pickets or pales.
7. 8.To tether to, or as to, a picket; as, to picket a horse.
8. 9.To guard, as a camp or road, by an outlying picket.
9. 10.To torture by compelling to stand with one foot on a pointed stake. [Obs.]