An external dependency is an input from an external source that is required before a task can proceed. This dependency frequently takes the form of an approval. For example:
A government agency must issue an operating license to a power plant before the facility can be turned on. The license is an external dependency.
A customer must review the progress of a project and give his approval before he will release funds to pay for the next summary task. The approval is an external dependency.
A building inspector must issue a permit before the wiring for a building will be approved. The permit is an external dependency.
A project manager needs to track external dependencies closely to ensure that a project is completed on schedule. This can be done with ongoing meetings to review the situation in regard to each of these dependencies.