Longman Dictionary of Contemporary English
The Collaborative International Dictionary
Picket \Pick"et\, n. [F. piquet, properly dim. of pique spear, pike. See Pike, and cf. Piquet.]
A stake sharpened or pointed, especially one used in fortification and encampments, to mark bounds and angles; or one used for tethering horses.
A pointed pale, used in marking fences.
[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.
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]
A military punishment, formerly resorted to, in which the offender was forced to stand with one foot on a pointed stake.
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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 position held and guarded by small bodies of men placed at intervals.
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A rope to which horses are secured when groomed.
Picketpin, an iron pin for picketing horses.
Wiktionary
n. 1 (context rare English) A line or rope held by one or many pickets, chiefly one used for tethering horses. 2 (context military rare English) A barrier or fortification formed by pickets; a stockade. 3 A boundary guarded by a picket, or unit, of soldiers. 4 (context US English) The boundary created by strike workers, generally at the workplace entrance, which other workers are asked not to pass.
WordNet
n. a line of people acting as pickets
Wikipedia
A picket line is a horizontal rope, along which horses are tied at intervals. The rope can be on the ground, at chest height (above the knees, below the neck), or overhead. The overhead form usually is called a high line.
A variant of a high line, used to tie a single horse, is a horizontal pole attached high on the side of a horse trailer. The attachment is designed so that the pole can be removed or folded against the trailer when not in use.
Usage examples of "picket line".
The other way you can do it is by spending your evenings going around trying to organize other people who will then spend their evenings at meetings, go out on a picket line, carry out a long struggle in which they'll be beaten up by the police and lose their jobs.
News of the Colonel's conversion had spread through the Brigade, and men were drifting toward the picket line to see the proof for themselves.
He was sure of one thing: he had to get out the door to get to the picket line by the time the construction crew got to the site.
I know that Sexton has to go there and be on the picket line like everyone else, but I hope you are being sensible and are not getting caught up in any of the fighting.
He was well away from the stable and close to the picket line when he heard a shout, and then a single shot that felt very close to his head.
He's been proposing some scheme or other and has crawled into the enemy's picket line at night….
Suddenly Moreta faced a scrawny, thin-legged, big-kneed, midbrown runner, standing all by itself at the end of a half-empty picket line.
The handler bobbed an approach, apologizing and bowing and generally so abasing himself that Moreta cut him short by remarking that worse things had come flying out of a picket line, and she was grateful it was no more than dirty water.
Suddenly Moreta faced a scrawny, thin-legged, big-kneed, mid- brown runner, standing all by itself at the end of a half-empty picket line.