Convergence order
Two functions f and g can be expressed by $$ \begin{equation} f(h) = O(g(h))\quad\text{as}\quad h\to 0, \end{equation} $$ if there exists a constant K such that $$ \begin{equation} |f(h)| \leq K |g(h)|\quad\text{for all}\quad h\ll 1. \end{equation} $$ Example: $$ \begin{equation} \sin(h) = O(h)\quad\text{because}\quad \sin(h) = h - \frac{h^3}{3!} + \cdots \end{equation} $$