abacus or impost block: The slab at the top of a capital between the capital and the architectural member above.
See also column, pier
Other parts of a column or pier: capital, shaft, column base
acanthus: A Mediterranean plant. The leaves are thick, fleshy, and scalloped. A stylization of the acanthus leaf began in Greek and Roman decoration, especially on the Corinthian capital.
See also Corinthian capital
aisle: Open area of a church parallel to the nave and separated from it by columns or piers.
See also nave
altar: In the Roman Church, a table at which the celebration of the Eucharist takes place. It is placed in the most prominent place in the church, usually at the east end, in the choir or sanctuary, facing the main entrance to the church.
See also choir, retable
alternation of support: A system of supports for an arcade or colonnade in which there are two different types of support. The alternation may be quite obvious, between one pier (strong support) and one column (weak support), or it may exist only in slight differences, such as in the treatment of the shafting on each pier.
See also arcade, colonnade, column, pier
ambulatory: A semicircular or polygonal aisle.  Usually an ambulatory leads around the east end of the choir; separating the choir from apses or chapels.
See also aisle, apse, choir, east end, hemicycle
applied or engaged column: A column which is attached to a wall so that only half of the form projects from the wall.
See also column
apse: A vaulted extension or projection, usually from a choir or chapel and generally circular or polygonal in shape.
Contrast with niche
arcade: A series of arches supported by columns or piers.
Contrast with colonnade
See also blind arcade, column, pier, spandrel
architrave: The lintel or flat horizontal member which spans the space between columns; in classical architecture, the lowest member of an entablature.

arch: A curved structural member spanning an opening or recess. The wedge shaped elements that make up an arch keep one another in palce and transform the vertical pressure of the structure above into lateral pressure.

Parts of an arch: keystone, soffit, spring line, springer, voussoir.
Measurements of an arch: rise, span.
Related architectural elements: arcade, cusp, hood moulding, spandrel, tympanum.
See also: intersecting arch, blind arch, relieving arch,
Shapes of arches.

archivolts: Bands or mouldings (moldings, Am.) surrrounding an arched opening.

See also: jamb, tympanum
atrium: An open courtyard at the entrance of a church, usually surrounded by covered aisles. The atrium of the Early Christian church was originally a place for the catechumens to wait during the celebration of the Eucharist.
Compare with cloister