'Picket line' definitions:

Definition of 'picket line'

noun

A line of people acting as pickets

From: GCIDE

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. [1913 Webster]
2. A pointed pale, used in marking fences. [1913 Webster]
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. [1913 Webster]
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] [1913 Webster]
5. A military punishment, formerly resorted to, in which the offender was forced to stand with one foot on a pointed stake. [1913 Webster]
6. A game at cards. See Piquet. [1913 Webster]
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. [1913 Webster]