Etymology
Advertisement
cline (n.)

1938, in biology, "a graded series of differences within a species," a back-formation from incline or from a Latinized form of Greek klinein "to slope, to lean" (from PIE root *klei- "to lean"). Earlier, but now obsolete, was a verb cline, from Middle English clinen "to bend, bow," from Old French cliner, from Latin clinare.

Related entries & more 
Advertisement
*klei- 

Proto-Indo-European root meaning "to lean."

It forms all or part of: acclivity; anticline; clemency; client; climate; climax; cline; clinic; clinical; clino-; clitellum; clitoris; decline; declivity; enclitic; heteroclite; incline; ladder; lean (v.); lid; low (n.2) "small hill, eminence;" matroclinous; patroclinous; polyclinic; proclitic; proclivity; recline; synclinal; thermocline.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit srayati "leans," sritah "leaning;" Old Persian cay "to lean;" Lithuanian šlyti "to slope," šlieti "to lean;" Latin clinare "to lean, bend," clivus "declivity," inclinare "cause to bend," declinare "bend down, turn aside;" Greek klinein "to cause to slope, slant, incline;" Old Irish cloin "crooked, wrong;" Middle Irish cle, Welsh cledd "left," literally "slanting").

Related entries & more 
thermocline (n.)
1897, from thermo- + -cline, from Greek klinein "to slope," from PIE root *klei- "to lean."
Related entries & more