Definition of half adverb from the Oxford Advanced American Dictionary

     

    half

     adverb
    adverb
    NAmE//hæf//
     
     
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  1. 1to the extent of half The glass was half full.
  2. 2partly The chicken was only half cooked. half-closed eyes I'm half inclined to agree.
  3. Idioms
    half again as much
     
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    an increase of 50% of the existing number or amount Spending on health is half again as much as it was in 2009.
    not nearly He's not half as rich as they say he is.
    not half bad(informal)
     
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    (used to show surprise) not bad at all; good It really isn't half bad, is it?
Grammarhalf / whole / quarterQuarter, half, and whole can all be nouns:Cut the apple into quarters. Two halves make a whole.Whole is also an adjective:I’ve been waiting here for a whole hour.Half is also a determiner:Half (of) the work is already finished. They spent half the time looking for a parking space. Her house is half a mile down the road.Note that you do not put a or the in front of half when it is used in this way:I waited for half an hour I waited for a half an hour.Half can also be used as an adverb:This meal is only half cooked.
See the Oxford Advanced Learner's Dictionary entry: half