O.E.
hyll, from P.Gmc.
*khulnis (cf. M.Du.
hille, Low Ger.
hull "hill," O.N.
hallr "stone," Goth.
hallus "rock," O.N.
holmr "islet in a bay," O.E.
holm "rising land, island"), from PIE base
*kel- "to rise, be elevated, to be prominent" (cf. Skt.
kutam "top, skull;" L.
collis "hill,"
columna "projecting object,"
culmen "top, summit,"
cellere "raise,"
celsus "high;" Gk.
kolonos "hill,"
kolophon "summit;" Lith.
kalnas "mountain,"
kalnelis "hill,"
kelti "raise"). Formerly including mountains, now usually confined to heights under 2,000 feet.
Hillock (1382) preserves M.E. dim. suffix
-oc. Phrase
over the hill "past one's prime" is first recorded 1950.
"In Great Britain heights under 2,000 feet are generally called hills; 'mountain' being confined to the greater elevations of the Lake District, of North Wales, and of the Scottish Highlands; but, in India, ranges of 5,000 and even 10,000 feet are commonly called 'hills,' in contrast with the Himalaya Mountains, many peaks of which rise beyond 20,000 feet." [OED]
[hil] 
l]