Definition:O Notation/Big-O Notation/Implied Constant

Definition
Let $f$ and $g$ be real functions such that $f$ is big-O of $g$.

From the definition of the limit of a function, it can be seen that this is also equivalent to:
 * $\exists c \in \R: c > 0, k \ge 0: \forall n > k, f \left({n}\right) \le c g \left({n}\right)$

For some fixed $k$ (appropriate to the function under consideration) the infimum of such $c$ is called the implied constant.