What is "complement of an arc"

Complement \Com"ple*ment\, n. [L. complementun: cf. F. compl['e]ment. See Complete, v. t., and cf. Compliment.]

1. That which fills up or completes; the quantity or number required to fill a thing or make it complete.

2. That which is required to supply a deficiency, or to complete a symmetrical whole.

   History is the complement of poetry.
   --Sir J. Stephen.

3. Full quantity, number, or amount; a complete set; completeness.

   To exceed his complement and number appointed him which was one hundred and twenty persons.
   --Hakluyt.

4. (Math.) A second quantity added to a given quantity to make it equal to a third given quantity.

5. Something added for ornamentation; an accessory. [Obs.]

   Without vain art or curious complements.
   --Spenser.

6. (Naut.) The whole working force of a vessel.

7. (Mus.) The interval wanting to complete the octave; -- the fourth is the complement of the fifth, the sixth of the third.

8. A compliment. [Obs.]
   --Shak.

   Arithmetical compliment of a logarithm. See under Logarithm.

   Arithmetical complement of a number (Math.), the difference between that number and the next higher power of 10; as, 4 is the complement of 6, and 16 of 84.

   Complement of an arc or Complement of an angle (Geom.), the difference between that arc or angle and 90[deg].

   Complement of a parallelogram. (Math.) See Gnomon.

   In her complement (Her.), said of the moon when represented as full.