Dependency relationships are used to model a wide range of dependent relationships between model elements in Use Case, Activity and Structural diagrams, and even between models themselves. You can create the Dependency from the Common page of the Toolbox. The Dependencies Package as defined in UML 2.1 has many derivatives, such as Realize, Deployment and Use. Once you create a Dependency you can further refine its meaning by applying a specialized stereotype.
OMG UML Specification:
The OMG UML specification (UML Superstructure Specification, v2.1.1, p.64) states:
A dependency is a relationship that signifies that a single or a set of model elements requires other model elements for their specification or implementation. This means that the complete semantics of the depending elements is either semantically or structurally dependent on the definition of the supplier element(s).