A recurring decimal is a decimal number that has a digit or a group of digits that repeat infinitely. For example, the fraction 1/3 equals 0.333..., where the digit 3 continues forever. This repeating part can be indicated by placing a bar over the repeating digit(s), like 0.3̅. Recurring decimals can be either simple or complex. A simple recurring decimal has one digit that repeats, such as 0.666... (which is 2/3), while a complex recurring decimal has multiple digits that repeat, like 0.142857142857... (which is 1/7). Understanding recurring decimals is important in mathematics, especially in fractions and number theory.