A reversible number is a number that, when its digits are reversed, results in a different number that is also a valid integer. For example, the number 21 is reversible because its reverse, 12, is also a valid number. Reversible numbers are often explored in number theory and can be interesting in various mathematical contexts. In addition to being different when reversed, reversible numbers can also be classified based on their properties, such as whether they are odd or even. For instance, if both a number and its reverse are odd, they are considered a special type of reversible number. This concept can be related to other mathematical topics like palindromic numbers and digit manipulation.