barrier

A structure that bars passage.

An obstacle or impediment.

A boundary or limit.

(grammar) A node (in government and binding theory) said to intervene between other nodes A and B if it is a potential governor for B, c-commands B, and does not c-command A.

(physiology) A separation between two areas of the body where specialized cells allow the entry of certain substances but prevent the entry of others.

(historical) The lists in a tournament.

(historical, in the plural) A martial exercise of the 15th and 16th centuries.