An arch is a structure that spans a space while supporting mass (e.g. a doorway in a stone wall). Arches appeared as early as the 2nd millennium BC in Mesopotamian brick architecture and their systematic use started with the Ancient Romans who were the first to apply the technique to a wide range of structures.

Elements of an arch:
Voussoir is a wedge-shaped element, typically a stone, (piatra) used in building an arch.
Keystone or Key is the architectural piece at the crown of a vault or arch which marks its apex, locking the other pieces into position.
Springing line is the start of the arch.
Rise is the distance from the springing line of the arch to the top of the arch.

Arch types

Lintel arch  Segmental arch
Semi-circular arch  Three centred arch
Parabolic arch  Lancet arch
Drop arch  Horseshoe arch
Ogee arch  

Drawings by Frank Gaylard from: Fisher, Roger C. 1992. Visual Lexicon of the South African Dwelling. Cape Town: Unibook. p, 10.