1640s, "act of receding, a going back," from French récession "a going backward, a withdrawing," and directly from Latin recessionem (nominative recessio) "a going back," noun of action from past-participle stem of recedere "to go back, fall back; withdraw, depart, retire," from re- "back" (see re-) + cedere "to go" (from PIE root *ked- "to go, yield").
The sense of "temporary decline in economic activity" was a fall-of-1929 coinage, probably a noun of action from recess (v.):
The material prosperity of the United States is too firmly based, in our opinion, for a revival in industrial activity — even if we have to face an immediate recession of some magnitude — to be long delayed. [Economist, Nov. 2, 1929]
Ayto ("20th Century Words") notes, "There was more than a hint of euphemism in the coining of this term."
It forms all or part of: abscess; accede; access; ancestor; antecede; antecedent; cease; cede; cession; concede; decease; exceed; excess; incessant; intercede; necessary; precede; predecessor; proceed; recede; recess; recession; secede; secession; succeed; success.
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit sedhati "to drive, chase away;" Avestan apa-had- "turn aside, step aside;" Latin cedere "to yield, give place; to give up some right or property," originally "to go from, proceed, leave;" Old Church Slavonic chodu "a walking, going," choditi "to go."
in law, "a recession of the sea or other body of water from the land," also the land thus discovered, 1670s, from Latin relictionem (nominative relictio), noun of action from past participle stem of relinquere "leave behind, forsake, abandon, give up," from re- "back" (see re-) + linquere "to leave" (from PIE *linkw-, nasalized form of root *leikw- "to leave").
Old English lang "having a great linear extent, that extends considerably from end to end; tall; lasting," from Proto-Germanic *langa- (source also of Old Frisian and Old Saxon lang, Old High German and German lang, Old Norse langr, Middle Dutch lanc, Dutch lang, Gothic laggs "long").
The Germanic words perhaps are from PIE *dlonghos- (source also of Latin longus "long, extended; further; of long duration; distant, remote," Old Persian darga-, Persian dirang, Sanskrit dirghah "long"), from root *del- (1) "long" (source also of Greek dolikhos "long," endelekhes "perpetual"). Latin longus (source of prolong, elongate, longitude, etc.) thus is probably cognate with, but not the source of, the Germanic words. The word illustrates the Old English tendency for short "a" to become short "o" before -n- (also retained in bond/band and West Midlands dialectal lond from land and hond from hand).
Also in Old English in reference to time, "drawn out in duration," with overtones of "serious." The old sense of "tall" now appears to be dialectal only, or obsolete. For long "during a long time" is from c. 1300. To be long on something, "have a lot" of it, is from 1900, American English slang. A long vowel (c. 1000) originally was pronounced for an extended time. Mathematical long division is from 1808. Sporting long ball is from 1744, originally in cricket. Long jump as a sporting event is attested from 1864. A long face, one drawn downward in expression of sadness or solemnity, is from 1786. Long in the tooth (1841 of persons) is from horses showing age by recession of gums (but not in this sense until 1870). Long knives, name Native Americans gave to white settlers (originally in Virginia/Kentucky) is from 1774, perhaps a reference to their swords. Long time no see, supposedly imitative of American Indian speech, is first recorded 1919 as Chinese English.