picket line

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A line or procession of people picketing a place of employment or otherwise staging a public protest.

from The Century Dictionary.

• noun A position held by an advance-guard of men stationed at considerable intervals.
• noun A rope to which cavalry and artillery horses are tied while being groomed.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun rare A line or rope held by one or many pickets, chiefly one used for tethering horses.
• noun military (rare) A barrier or fortification formed by pickets; a stockade.
• noun A boundary guarded by a picket, or unit, of soldiers.
• noun US The boundary created by striking workers, generally at the workplace entrance, which other workers are asked not to pass.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a line of people acting as pickets