It is the initial observation or value of a measurable quantity at the time of diagnosis or assessment (before an intervention) against which later tests will be compared.
The baseline and the response values refer to the same individual or system.

Baseline assessment: it is performed during the design phase of a surveillance plan of action. It provides information on the existing situation, forms the basis for the development of the plan of action, and provides baseline data against which prospective changes in the surveillance system are progressively assessed or measured.

Baseline data: it is collected at the outset of implementation of a surveillance system or of strengthening activities, or a set of indicators that have been identified to monitor and evaluate the performance of a surveillance and response system. For example, the baseline mortality rate (or non-crisis mortality rate) is mortality rate before the crisis.

Adapted from the World Health Organization. Definitions. Internet. Accessed on May 24, 2016.