A "Requirements Baseline" is a "snapshot" of items (such as requirements, features, use cases, etc) at a certain point in time.

You may find it useful to create a baseline at each significant milestone during a project - such as:

• When requirements are ready for initial review,
• When requirements are shared with another team,
• When requirements are frozen for a release,
• etc...

The activity of creating and using baselines is usually referred to as "Requirements Baselining".