de·cline / diˈklīn/ • v. 1. [intr.] become smaller, fewer, or less; decrease. ∎  diminish in strength or quality; deteriorate. [as adj.] (declining) declining educational standards. 2. [tr.] politely refuse (an invitation or offer). ∎  politely refuse to do something: the company declined to comment.3. [intr.] (esp. of the sun) move downward. ∎ archaic bend down; droop.4. [tr.] (in the grammar of Latin, Greek, and certain other languages) state the forms of (a noun, pronoun, or adjective) corresponding to cases, number, and gender.• n. [in sing.] a gradual and continuous loss of strength, numbers, or quality. ∎  a fall in value or price. ∎ archaic the gradual setting of the sun. ∎ archaic any disease in which bodily strength gradually fails, esp. tuberculosis.DERIVATIVES: de·clin·a·ble adj.de·clin·er n.

decline turn aside, deviate (trans. and intr.); bend or go down; (gram.) inflect XIV; turn aside or away from XV. — (O)F. décliner — L. dēclīnāre, f. DE- 2 + clīnāre bend, cogn. with Gr. klinein bend; cf. LEAN 2.
Hence decline sb. XIV. So declination (astron.) XIV; †(gram.) declension XV; turning aside or down XVI. — L.