Odd and even are mathematical terms. If a number is 1,3,5,7,9,11 etc it's an odd number and if a number is 0,2,4,6,8,10 etc it's an even number.
In short, odd numbers cannot be divide evenly into a group of two, while an even number can be (For example, 5 divide by 2 equals 2.5 which isn't a whole number, however 8 can be divide by 2 to get 4 which is a whole number and so therefore is evenly divided).
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