picket line

noun4

1. •
   A boundary guarded by a picket (unit of soldiers).
2. •
   A boundary created by workers participating in a strike, generally at the workplace entrance, which other workers are asked not to pass.

   • RMT union members form a picket line outside Birmingham New Street on July 27, as part of a national dispute over jobs, pay and conditions. — 2022
3. •
   (rare) A line or rope held by one or many pickets, chiefly one used for tethering horses.
4. •
   (rare) A barrier or fortification formed by pickets; a stockade.