An ancient technical term applied in masonry and metal work to those portions which are sunk beneath the surface, as in inscriptions where the ground is sunk round the letters so as to leave the letters or ornament in relief.

* From 1911 Encyclopedia Britannica

Mortar

Mortar is a material used in masonry to fill the gaps between blocks in construction and bind the blocks together. The blocks may be stone, brick, breezeblocks (cinder blocks), etc. Mortar is a mixture of
* sand,
* a powdered adhesive such as cement,
* and water
and is applied as a paste which then dries hard.