Origins
The word 'recognize' entered English in the early 15th century from Old French 'reconois-' (the stem of 'reconoistre,' to know again, to identify, to acknowledge), which descended from Latin 'recognōscere' (to know again, to recall to mind, to investigate, to review). The Latin verb is composed of 're-' (again, back) + 'cognōscere' (to get to know, to become acquainted with), itself from 'co-' (together) + 'gnōscere' (earlier 'nōscere,' to come to know), from PIE *ǵneh₃- (to know).
The PIE root *ǵneh₃- is one of the fundamental vocabulary items of the Indo-European family. Through the Germanic branch, it produced Old English 'cnāwan' → modern 'know,' along with 'knowledge,' 'acknowledge,' 'can' (originally 'to know how'), 'cunning' (originally 'knowing'), 'canny,' 'uncouth' (originally 'unknown'), and 'ken' (Scottish/dialectal, 'to know'). Through Latin 'gnōscere/nōscere,' it gave 'cognition,' 'cognitive,' 'incognito' (unknown), 'reconnaissance,' 'recognize,' 'prognosis' (foreknowledge), 'diagnose' (to know through, to distinguish), 'ignore' (from 'ignōrāre,' to not know), 'ignorant,' 'noble' (from 'nōbilis,' knowable, notable, famous), 'note,' 'notion,' 'notorious,' and 'annotate.'
Through Greek 'gnṓsis' (γνῶσις, knowledge), the root produced 'gnosis,' 'gnostic,' 'agnostic,' 'prognosis,' and 'diagnosis.' Through Sanskrit 'jñā' (to know), it produced 'jñāna' (knowledge, wisdom) — a central concept in Hinduism and Buddhism — and contributed to the title 'Jnana Yoga' (the path of knowledge).
Word Formation
The English verb 'recognize' has developed several distinct senses that have diverged over the centuries. The core sense remains 'to identify someone or something previously known' — the 're-' prefix emphasizes knowing again, re-cognizing. A second major sense is 'to acknowledge formally' — to recognize a government, to recognize a speaker, to recognize an achievement. This sense comes from the medieval legal meaning: in law, to 'recognize' was to formally declare or acknowledge something before a court. The legal noun 'recognizance' (an obligation entered before a court) preserves this usage.
The military and diplomatic term 'reconnaissance' (a survey to gain information) is essentially the same word, re-borrowed from French in the 18th century with French pronunciation intact. The French verb 'reconnaître' retains the full range of meanings: to identify, to acknowledge, to scout (as in military reconnaissance), and to be grateful (the noun 'reconnaissance' in its other French sense means 'gratitude').
The spelling of 'recognize' has varied considerably. The '-ize' ending reflects the influence of Latin '-izāre' (from Greek '-izein'), but the British variant 'recognise' (with '-ise') is also standard. American English overwhelmingly prefers '-ize.' The word's pronunciation has also shifted — the 'g' before 'n' in 'recognize' was originally pronounced, as it still is in 'cognition' and 'recognition,' but it has become silent in casual speech for many speakers of the verb form.