Borrowed from French gnomon, or directly from its etymon Latin gnōmōn, or directly from its etymon Ancient Greek γνώμων, from γιγνώσκω, ultimately from Proto-Indo-European *ǵneh₃-; the word is thus related to know.

The geometry sense (sense 4) is from the resemblance of the plane figure to a carpenter’s square.

• (RP) IPA: /ˈnəʊˌmɒn/, /ˈnəʊ.mən/
• (America) IPA: /ˈnoʊˌmɑn/, /ˈnoʊ.mən/

gnomon (plural gnomons)

1. An object such as a pillar or a rod that is used to tell time by the shadow it casts when the sun shines on it, especially the pointer on a sundial. [from mid 16th c.]

   Synonyms: cock, style

2. An object such as a pillar used by an observer to calculate the meridian altitude of the sun (that is, the altitude of the sun when it reaches the observer's meridian), for the purpose of determining the observer's latitude.
3. The index of the hour circle of a globe.
   • 1809, William Nicholson, “Index of a globe ↗”, in The British Encyclopedia, or Dictionary of Arts and Sciences; […], volume III (E … I), London: Printed by C[harles] Whittingham, […]; for Longman, Hurst, Rees, and Orme, […], →OCLC ↗, column 2:

     Index of a globe, the little style or gnomon, which being fixed on the pole of the globe, and turning round with it, points out the hours upon the hour circle.

4. (geometry) A plane figure formed by removing a parallelogram from a corner of a larger parallelogram.
   • 1728, E[phraim] Chambers, “Gnomon”, in Cyclopædia: Or, An Universal Dictionary of Arts and Sciences; […] In Two Volumes, volumes I (A–H), London: […] James and John Knapton [et al.], →OCLC ↗, page 164 ↗, column 2:

     If a Parallelogram be divided into four leſſer ones, by two Lines interſecting each other; and one of theſe Parallelograms be retrench'd, or taken away; the other three will make a Gnomon, ordinarily call'd a Square.

5. (mathematics, by extension) A number representing the increment between two figurate number.

• French: gnomon, style
• German: Gnomon
• Italian: gnomone, stilo
• Portuguese: gnômon
• Russian: гно́мон
• Spanish: estilo

• French: gnomon