Spec freeze
Beginner
The point where the team agrees to stop changing the plan, so building can start in earnest.
Novice
A milestone after which the requirements change only through a formal change request that weighs the effect on schedule, cost and the other requirements.
Expert
In practice a series: requirements freeze, architecture freeze, then feature freeze in the design code. Each later change needs an impact analysis across design code, verification, physical layout, software and manufacturing test.