An odd number is any integer (a whole number) that cannot be divided by 2 evenly. If you divide a number by 2 and there is a remainder left, the number is odd. If you divide by 2 and there is not a remainder left, then the number is even.
Let's try that calculation...
100 ÷ 2 = 50
Notice the answer is a whole number and doesn't have a remainder? That means the number is even, as it can cleanly be divided by 2, with no remaining remainder.
Any number that ends in 1, 3, 5, 7 or 9 is an odd number. This is because the number cannot be divided by 2 evenly.
Here are more examples of even and odd numbers in sequence: