baseline

in chromatography
The portion of the chromatogram recording the detector response when only the mobile phase emerges from the column.
Source:
PAC, 1993, 65, 819 (Nomenclature for chromatography (IUPAC Recommendations 1993)) on page 834
See also:
PAC, 1990, 62, 2167 (Glossary of atmospheric chemistry terms (Recommendations 1990)) on page 2176