Milestone Definition

A milestone is a scheduled event signifying the completion of a major deliverable or a set of related deliverables. A milestone has zero duration and no effort -- there is no work associated with a milestone. It is a flag in the workplan to signify some other work has completed.

Usually a milestone is used as a project checkpoint to validate how the project is progressing and revalidate work. Milestones are also used as high-level snapshots for management to validate the progress of the project. In many cases there is a decision to be made at a milestone.

Other Definitions

Assumption
Business Case
Critical Path
Executive Sponsor
Milestone
Project Lifecycle
Project Management Institute
Project Objective
Project Planning

Project Portfolio Management
Project Task
Project Team
Risk
ROI
Scope Creep
Stakeholder
Steering Committee

I've also put together a short online course that describes project management fundamentals if you're interested in learning more about this profession.

References
Tom Mochal and Jeff Mochal, Lessons in Project Management