a curved symmetrical structure spanning an opening and supporting a ceiling, roof, wall, bridge, or other part above it

something, e.g. the vaulted bony structure of the foot, resembling an arch in form or function

an archway

to form or bend (something) into an arch

The cat arched its back

to span (an opening) or provide (a wall or other structure) with an arch

to form an arch

archaic preeminent; exceptional

the most arch deed of piteous massacre — Shakespeare