Arch. n.s. [arcus, Lat.]

1. Part of a circle, not more than the half.

   The mind perceives, that an arch of a circle is less than the whole circle, as clearly as it does the idea of a circle. Locke.

2. A building in form of a segment of a circle, used for bridges and other works.

   Ne'er through an arch so hurried the blown tide,
   As the recomforted through the gates. Shakesp. Coriolanus.

   Let Rome in Tiber melt, and the wide arch
   Of the rais'd empire fall! here is my space. Shakespeare's Antony and Cleopatra.

   The royal squadron marches,
   Erect triumphal arches
   For Albion and Albanius. Dryden's Albion.

3. The sky, or vault of heaven.

                       Hath nature given them eyes
   To see this vaulted arch, and the rich cope
   Of sea and land. Shakesp. Cymbeline.

4. [from ἄρχος.] A chief: obsolete.

                   The noble duke, my master,
   My worth arch and patron comes to night. Sh. King. Lear.