(construction) A double wall of masonry connected by an air space.

(history) A wall that parted the city of Berlin until 1989, constructed by the German Democratic Republic to keep East Germans from escaping to West Berlin.

(politics) Any barrier designed to keep people from crossing a border, e.g. the one proposed to keep people from crossing from Mexico into the United States.

Short for Great Wall of China. / (astronomy) Either of two identified areas containing hundreds of galaxies.

plural of wall clock

To separate with a wall. / (figuratively, by extension) To isolate completely.

(transitive) To seal with a wall.

(idiomatic, jargon) An information barrier within an organization, designed to ensure internal confidentiality; frequently used to prevent conflicts of interest.

(idiomatic) crazy, mad

(civil engineering) A structure inserted in the ground and used to maintain an excavation open or to isolate portions of contaminated ground. It can be built of any material from steel to reinforced concrete.