word-forming element meaning "not, opposite of, without" (also im-, il-, ir- by assimilation of -n- with following consonant, a tendency which began in later Latin), from Latin in- "not," cognate with Greek an-, Old English un-, all from PIE root *ne- "not."
In Old French and Middle English often en-, but most of these forms have not survived in Modern English, and the few that do (enemy, for instance) no longer are felt as negative. The rule of thumb in English has been to use in- with obviously Latin elements, un- with native or nativized ones.
late 14c., "a giving heed, active direction of the mind upon some object or topic," from Old French attencion and directly from Latin attentionem (nominative attentio) "attention, attentiveness," noun of action from past-participle stem of attendere "give heed to," literally "to stretch toward," from ad "to, toward" (see ad-) + tendere "stretch," from PIE root *ten- "to stretch."
Rare in English before 17c. Meaning "consideration, observant care" is from 1741; that of "civility, courtesy" is from 1752. Meaning "power of mental concentration" is from 1871. It is used with a remarkable diversity of verbs (pay, gather, attract, draw, call, etc.). As a military cautionary word before giving a command, it is attested from 1792. Attention span is from 1903 (earlier span of attention, 1892). Related: Attentions.