Definition:Fractional Part

Definition
Let $x \in \R$ be any real number.

Let $\left \lfloor {x}\right \rfloor$ be the floor function of $x$.

Then the quantity $x - \left \lfloor {x}\right \rfloor$ is called the fractional part of $x$, and is frequently denoted:
 * $\left\{{x}\right\} := x - \left \lfloor {x}\right \rfloor$

Beware, of course, not to get $\left\{{x}\right\}$ confused with the singleton set containing $x$.

Also see
From Real Number minus Floor, we have that $0 \le \left\{{x}\right\} < 1$, or $\left\{{x}\right\} \in \left[{0 \,.\,.\, 1}\right)$.

Compare with the definition of modulo 1:
 * $x \bmod 1 = \left\{{x}\right\}$