The definition of a rational number is: Any number that can be made by building a fraction using whole numbers.
You can make 0.123 that way. Here's the fraction: 123/1000
It's also handy to know that any number that's completely written down using digits ... and a fraction bar or a decimal point if they're needed ... is a rational number. This is not a definition, and it doesn't cover all rational numbers, but it's handy sometimes.
It's also handy to know that a decimal that has a set of digits after the decimal point that repeats forever is a rational number, even though you can't completely write it down (because it never ends).
