"having unequal sides," in mathematics, 1680s, from Late Latin scalenus, from Greek skalenōs "uneven, unequal, odd (numbered)," as a noun, "triangle with unequal sides" (trigōnon skalēnon), from skallein "to chop, hoe, dig, stir up" (compare its derivative, skalops "a mole"), from PIE root *skel- (1) "to cut."

in mathematics, "a quantity or symbol to be operated on," 1886, from Latin operandum, neuter gerundive of operari "to work, labor" (in Late Latin "to have effect, be active, cause"), from opera "work, effort," related to opus (genitive operis) "a work" (from PIE root *op- "to work, produce in abundance").

"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").

1550s, originally in mathematics, from converse (adj.). From 1786 as "thing or action that is the exact opposite of another." As an example, Century Dictionary gives "the hollows in a mold in which a medal has been cast are the converse of the parts of the medal in relief." Chaucer has in convers, apparently meaning "on the other side."

1798, as a term in mathematics, "pertaining to modulation," from French modulaire or directly from Modern Latin modularis, from Latin modulus "a small measure," diminutive of modus "measure, manner" (from PIE root *med- "take appropriate measures"). Meaning "composed of interchangeable units" is recorded by 1936.

1788, in linguistics, "two letters used to represent one sound," from Greek di- "twice" (from PIE root *dwo- "two") + -graph "something written," from Greek graphe "writing," from graphein "to write, express by written characters," earlier "to draw, represent by lines drawn" (see -graphy). In mathematics (by 1955) it is a contraction of directed graph. Related: Digraphic.

"character or state of being in proportion," 1560s, from French proportionalité (14c.) or directly from Medieval Latin proportionalitas, from proportio "comparative relation, analogy" (see proportion (n.)). The word was used in Middle English (proporcionalite) in mathematics in reference to geometrical ratios (mid-15c.).

"the number two, two units treated as one," 1670s, from Latin dyad-, stem of dyas, from Greek dyas "the number two, a group of two," from duo "two" (from PIE root *dwo- "two"). Specific sense in chemistry ("a bivalent element") is by 1865; also used in biology, poetics, mathematics. Related: Dyadic.