A given is some detail or fact that is known to be true. Your mother might say, "It's a given that I'll always love you, no matter how annoying you are."
Something's a given if it's an established situation that you're sure of. It's a given that a first class airplane ticket will cost a lot more than a seat in coach, and it's also a given that the earth orbits the sun. It can also be an adjective: "After considering the given facts, mark an answer on the multiple-choice test." Given comes from the verb give, which has an Old English root of giefan, "to bestow, allot, or commit."