Definition of 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, springer (springing, spring line; see also stilted arch), voussoir. 

Measurements of an arch: rise, span. 

Shapes of arches: round, horseshoe, pointed, cusped, ogee, depressed (four-centred). 

Related architectural elements: arcade, cusp, hood moulding, spandrel, tympanum. 

See also: intersecting arch, blind arch, relieving arch 
 

springer	voussoirs	keystone	rise and span
soffit	relieving arch	arcade of round arches	blind arches
Lindisfarne Gospels:  canon table in a  round-arched frame	round arched windows	arcade and clerestorey  of round arches	pointed arched arcade round-arched clerestorey
crossing arch, arches on transept ends, gallery arches all round arches	narthex, west window, tower, all  round arches	arcade, gallery clerestorey,  all round arches	round-arched door  and window, decorated with chevron mouldings and hood mouldings
horseshoe arch	horseshoe arches	intersecting arches	intersecting hanging  arches
cloister of pointed arches	cusped arched frame	pointed arches for  windows detached arcading. and cusped hanging arches
narthex with  pointed arches	vaulting and arcading  of pointed arches	blind arcading pointed arches with  angels in the spandrels, enclosing quatrefoils and paired  cusped arches	spandrel
ogee arches	cusped ogee arches	depressed arch ..	depressed arch  with ogee arched gable