An Etymological Dictionary of Astronomy and Astrophysics

1a) To become decomposed.

1b) Of a radioactive nucleus, to disintegrate spontaneously into one or more different nuclei, accompanied by the emission of → alpha particles, → beta particles, → positrons, and/or → gamma rays.

2a) Decomposition.

2b) The → transition of a system from an → excited state to a less excited one. Radiative decay refers to the process when the energy difference between the states is taken away by radiation. Likewise, collisional decay is when the energy difference is removed by a target during a collision.

2c) Of a spacecraft, a gradual decrease in the radius of its orbit over time, caused by aerodynamic drag of the atmosphere and other forces. The rate of orbit decay rises as the spacecraft falls and encounters increasing atmospheric density, eventually resulting in reentry.
→ alpha decay; → beta decay; → dark matter decay; → decay chain; → decay constant; → decay mode; → decay product; → decay time; → inverse beta decay; → Ohmic decay time; → orbit decay; → radioactive decay; → turbulence decay.

Etymology (EN): From O.Fr. decair, from V.L. *decadere “to fall off,” from L. cadere “to fall,” PIE base *kad- “to fall” (cf. Pers. Gilaki katan “to fall,” ba.ka.tam “I fell,” dakatan “to fall (in a marsh, in a pit),” vakatan “to fall from tiredness, be exhausted,” fakatan “to fall from (lose) reputation,” Pers. Laki: katen “to fall,” kat “he fell,” beko “fall!,” Pers. Tabari: dakətə “fallen,” dakətən “to crash down,” dakət.gu “stray cow,” Arm. chacnum “to fall”).

Etymology (PE): Tabâhidan, verbal form of tabâhi, noun form of tabâh “spoiled, ruined, destroyed,” Mid.Pers. tapâh “spoiled, destroyed.” Maybe related to Mod.Pers. tâb “affliction, pain, torment; heat, burning,” tab “fever,” tâbidan, tâftan “to shine,” tafsidan “to become hot,” Av. tāp-, taf- “to warm up, heat,” tafsat “became hot,” tāpaiieiti “to create warmth,” cf.
Skt. tap- “to spoil, injure, damage; to suffer; to heat, be/become hot,” tapati “burns,” L. tepere “to be warm,” tepidus “warm,” PIE base *tep- “warm.”