A course is a continuous horizontal layer of similarly-sized building material one unit high, usually in a wall. The term is almost always used in conjunction with unit masonry

Masonry

Masonry is the building of structures from individual units laid in and bound together by mortar; the term masonry can also refer to the units themselves. The common materials of masonry construction are brick, stone, marble, granite, travertine, limestone; concrete block, glass block, stucco, and...

 such as brick

Brick

A brick is a block of ceramic material used in masonry construction, usually laid using various kinds of mortar. It has been regarded as one of the longest lasting and strongest building materials used throughout history.-History:...

, cut stone, or concrete masonry units ("concrete block").


Stretcher - The typical course style, masonry units are laid with their face parallel to the wall and the long dimension outwards.
Header - a course with the face of the unit parallel to the wall, and the small dimension outwards, often used to interlock two adjacent wythes.

Rowlock - A course with the long dimension parallel to the wall and the small face looking outward.