what is a rational number?

A rational number is any number that can be written as a common fraction.

As a decimal fraction, it either terminates (as 6, -3 1/2, and 7/20 do) or it infinitely repeats (as 1/3, 5 3/7, and -4 5/9 do).

Technically, a rational number is any number that can be put into the form  a/b,  where a and b are both integers and b ≠ 0.

The integers form this set:   { ..., -4, -3, -2, -1, 0, 1, 2, 3, 4, ...}.