The Operator Norm on the Set of Bounded Linear Operators
 Definition: Let $(X, \| \cdot \|_X)$ and $(Y, \| \cdot \|_Y)$ be normed linear spaces and let $T$ be a linear operator from $X$ to $Y$. The Operator Norm of $T$ is defined to be $\| T \|_{\mathrm{op}} = \inf \{ M : \| T(x) \|_Y \leq M \| x \|_X, \: \forall x \in X \}$.
Later on The Space of Bounded Linear Operators page we will see that $\mathcal B(X, Y)$ with the operator norm $\| \cdot \|_{\mathrm{op}}$ is a normed linear space.
 Proposition: Let $(X, \| \cdot \|_X)$ and $(Y, \| \cdot \|_Y)$ be normed linear spaces and let $T$ be a bounded linear operator from $X$ to $Y$. Then: a) $\displaystyle{\| T \|_{\mathrm{op}} = \sup_{x \in X, x \neq 0} \left \{ \frac{\| T(x) \|_Y}{\| x \|_X} \right \}}$. b) $\displaystyle{\| T \|_{\mathrm{op}} = \sup_{x \in X, \| x \| = 1} \left \{ \| T(x) \|_Y \right \}}$.