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Okay (also spelled OK, O.K.) is a colloquial English word denoting approval, assent, or acknowledgment. "Okay" has frequently turned up as a loanword in many other languages. As an adjective, "okay" means "adequate," "acceptable" ("this is okay to send out"), "mediocre" often in contrast to "good" ("the food was okay"); it also functions as an adverb in this sense. As an interjection, it can denote compliance ("Okay, I will do that"), or agreement ("Okay, that's good"). As a grammatical particle it does not modify any other particular word, but rather reinforces the general point being made, particularly if that point is being called into question. And so, for example, a response to “So the accident kept him from going to the reunion?” might be “Oh, he went to it okay, but he had bruised ribs and his car was a wreck.” In this case “okay” does not modify him or his going anywhere; it is a particle emphasizing the point that is being questioned. As a noun and verb it means "assent" ("The boss okayed the purchase"). The origins of "okay" are not known with certainty, and have been the subject of much discussion over the years. x

( Source: Wikipedia )

Note: Some say that American regiments used to raise a sign reading:OK; meaning Zero Killed; i.e. every thing is alright! But this piece of information is not sure. x