Definition:Uniform Continuity

Let $$I \subseteq \R$$ be a real interval.

A function $$f: I \to \R$$ is said to be uniformly continuous on $$I$$ if for every $$\epsilon > 0$$ there exists $$\delta > 0$$ such that the following property holds: for every $$x,y \in I$$ such that $$\vert x - y \vert < \epsilon$$ it happens that $$\vert f(x) - f(y) \vert < \delta.$$

A more formal way of stating this is the following: $$f:I \to \R$$ is uniformly continuous if the following property holds:

\forall \epsilon > 0 \quad \exists \delta > 0 \quad \text{ such that } \quad \Big(   x,y \in I \text{ and } \vert x - y \vert < \epsilon    \Rightarrow    \vert f(x) - f(y) \vert < \delta.  \Big) $$

Relationship to other concepts

 * 1) The property that $$f$$ is uniformly continuous on $$I$$ is stronger than that of being continuous on $$I$$: intuitively, continuity on an interval means that for each fixed point $$x$$ of the interval, the value of $$f(y)$$ is near $$f(x)$$ whenever $$y$$ is close to $$x$$, but how close you need to be in order for $$\vert f(x) - f(y) \vert$$ to be less than a given number may depend on the point $$x$$ you pick on the interval. Uniform continuity on an interval means that this can be chosen in a way which is independent of the particular point $$x$$. See the proof of this fact for a more precise explanation.
 * 2) The property that $$f$$ is uniformly continuous on $$I$$ is weaker than the property that $$f$$ is absolutely continuous on $$I$$.

Compare

 * The difference between convergence and uniform convergence.