Akademik

1. a small military force that serves as a first line of defense; if they become engaged in hostilities it will trigger the intervention of stronger military forces

2. a wire stretched close to the ground that activates something (a trap or camera or weapon) when tripped over

1. : a low-placed wire used to discourage trespassing on lawns or grass

2. : a wire concealed close to the ground and used especially in military operations to actuate a warning signal or set off an explosive device usually when pulled or moved

trip wire,

a wire which, when pulled releases a catch and starts a process, such as the explosion of a mine or the sounding of an alarm.