What is Product Baseline ?

Product baseline is  the documentation describing:

All of the necessary functional and physical characteristics of a configuration item.

The selected functional and physical characteristics designated for production acceptance testing.

Tests necessary for deployment/installation, operation, support, training, and disposal of the configuration item.

The purpose of a baseline is to provide:

Measurable progress points within the system development lifecyclebasis for change control in subsequent project phases.
 stable reference for future work.
 Intermediate and final points for assessing the fitness for purpose of project work products.

Baselines are defined by:

The names of the physical item(s) which constitute the baseline (e.g. design document, code unit).
The point in the project where the baseline is expected to be complete (e.g. at the end of a project phase).
The method to be adopted to approve baseline items.
The individuals responsible for approving baseline items (approval authorities). 

Effective baselines have the following characteristics:

A baseline must be associated with the production and formal approval of a physical deliverable such as a document or hardware/software component.
All items associated with a baseline must be placed under formal change control.