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...
123 ÷ 2 = 61.5
Notice the number is not a whole number and has a remainder (the .5)? That means the number is an odd number because it cannot be divided by 2 without a 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: