late 13c., ile, from Old French ile, earlier isle, from Latin insula "island," a word of uncertain origin.

Perhaps (as the Ancients guessed) from in salo "(that which is) in the (salty) sea," from ablative of salum "the open sea," related to sal "salt" (see salt (n.)). De Vaan finds this "theoretically possible as far as the phonetics go, but being 'in the sea' is not a very precise description of what an island is; furthermore, the Indo-Europeans seem to have indicated with 'island' mainly 'river islands.' ... Since no other etymology is obvious, it may well be a loanword from an unknown language." He proposes the same lost word as the source of Old Irish inis, Welsh ynys "island" and Greek nēsos "island." The -s- was restored first in French, then in English in the late 1500s.

1864, coined from Greek isos "equal" (see iso-) + baros "weight" (from PIE root *gwere- (1) "heavy"). Line connecting places with the same barometric pressure at the same time. Related: Isobaric.

"having the heads of the principal figures at about the same level," 1893, from Greek isokephalos "like-headed," from isos "equal" (see iso-) + kephalē "head" (see cephalo-). The rule or principle of ancient Greek art that makes figures standing and on horseback have heads carved on nearly the same level.