"having no angle," 1846, from Greek agonos, from a- "not" (see a- (3)) + -gonos "angled," from gōnia "angle, corner" (from PIE root *genu- (1) "knee; angle"). In reference to the imaginary line on the earth's surface connecting points where the magnetic declination is zero.

instrument for measuring solid angles, 1766, from Greek gōnia "corner, angle" (from PIE root *genu- (1) "knee; angle") + -meter. Related: Goniometry.

1560s, from French heptagon, from Greek heptagonon, from hepta "seven" (see septi-) + gōnia "angle, corner" (from PIE root *genu- (1) "knee; angle"). Related: Heptagonal.

"branch of mathematics that deals with relations between sides and angles of triangles," 1610s, from Modern Latin trigonometria (Barthelemi Pitiscus, 1595), from Greek trigonon "triangle" (from tri- "three" (see tri-) + gōnia "angle, corner" (from PIE root *genu- (1) "knee; angle") + metron "a measure" (from PIE root *me- (2) "to measure").

1560s, from Latin hexagonum, from Greek hexagonon, neuter of hexagonos "six-cornered, hexagonal," from hex "six" (see hexa-) + gōnia "angle, corner" (from PIE root *genu- (1) "knee; angle").

"plane figure having ten sides and angles," 1630s, from Modern Latin decagonum, from Greek dekagonon, from deka "ten" (from PIE root *dekm- "ten") + gōnia "corner, angle" (from PIE root *genu- (1) "knee; angle"). Related: Decagonal.

"pertaining to or depending upon the use of right angles," 1570s, from French orthogonal, from orthogone, from Late Latin orthogonius, from Greek orthogonios "right-angled," from ortho- "straight" (see ortho-) + gōnia "angle, corner" (from PIE root *genu- (1) "knee; angle"). Related: Orthogonally; orthogonality.

in trigonometry, "the tangent of the complement of a given angle," a contraction of co. tangent, abbreviation of complement + tangent (n.).