A unit or units leading the way of advancing or marching troops, guarding them from ambushes, reconnoitering and pursuing the enemy, etc. / An individual soldier in the same role.

(idiomatic, cricket) (For a new batsman, with help from the umpire) to mark a point on the popping crease in front of his wicket so that he knows where it is behind him; to guard.

of, or relating to a rearguard / of, or relating to resistance in politics etc

A physical barrier to protect something from unwanted contact, especially a shield to prevent accidental tripping of an emergency shutdown or power switch.

The organisation or officer enforcing maritime law and policing the seas within territorial waters.

A mesh screen placed in front of a fire to prevent sparks escaping.

Not vigilant; unprepared; inattentive. / Unexpectedly or by surprise.

A decommissioned aircraft, artillery piece, or tank, displayed near the entrance to an airport or military base.

A person who is employed to provide security for people or property.

(Canada, US, slang, offensive) A person employed to look after people with mental disabilities.