A specification or product that has been formally reviewed and agreed upon, that thereafter serves as the basis for further development, and that can be changed only through formal change control procedures. (ISO/IEEE 2008)
ISO/IEEE. 2008. Systems and Software Engineering — Software Life Cycle Processes. Geneva, Switzerland: International Organization for Standards (ISO)/Institute of Electrical & Electronics Engineers (IEEE) Computer Society, ISO/IEEE 12207:2008(E).
There are 7 definitions of baseline in the standards covered by the IEEE sevocab database. The included definition seems most appropriate for SEBoK use. See http://www.computer.org/sevocab for more detail.
The SEVOCAB entry provides 7 definitions followed by this note: A baseline should be changed only through formal configuration management procedures. Some baselines may be project deliverables while others provide the basis for further work. Baselines, plus approved changes from those baselines, constitute the current configuration identification.