An Etymological Dictionary of Astronomy and Astrophysics

1a) General: The way in which several items are arranged, as an indication of their relative importance or size or when each will be dealt with.

1b) Optics: → order of interference.

1c) Math.: The number of rows or columns of a
→ square matrix or → determinant.

1d) Math.: The highest → derivative appearing in a given → differential equation. For example,
d²y/dx² + 2y (dy/dx) - 3 = 0
is a differential equation of order two.

2. To arrange, regulate, or dispose things in their proper places. See also: → disorder, → first-order differential equation, → first-order partial derivative, → first-order spectrum, → interference order, → order of a tensor, → order of interference, → order of magnitude.

Etymology (EN): From O.Fr. ordre, from earlier ordene, from L. ordinem (nominative ordo) “row, rank, arrangement.”

Etymology (PE): Râyé, noun related to râyânidan “to regulate, set in order,” from Mid.Pers. râyânīdan “to arrange, organize,” from rây-
(Mod.Pers. ârây-, ârâyeš, ârâyidan “to arrange, adorn”),
ârây-, ârâstan “to arrange, adorn;” O.Pers. rād- “to prepare,” rās- “to be right, straight, true,” rāsta- “straight, true” (Mod.Pers. râst “straight, true”); Av. rāz- “to direct, put in line, set,” razan- “order;” Gk. oregein “to stretch out;” L. regere “to lead straight, guide, rule,” p.p. rectus “right, straight;” Skt. rji- “to make straight or right, arrange, decorate;” PIE base *reg- “move in a straight line;” see also → direct.