Definition of picket lines:
(n) :
A boundary created by workers participating in a strike, generally at the workplace entrance, which other workers are asked not to pass.
(n) :
(military) A boundary guarded by a picket (unit of soldiers).
(n) :
(military, rare) A barrier or fortification formed by pickets; a stockade.
(n) :
(rare) A line or rope held by one or many pickets, chiefly one used for tethering horses.