: a main line taken as or representing a base: such as
a:base 6b; specifically: a line extending east and west from a chosen reference point on a principal meridian and forming with the meridian a pair of coordinate axes for locating township and section corners —used in U.S. public-land surveying
b: the line joining the base piece and the base point in artillery fire
c: the lowest horizontal line in a profile drawing of a ship used as a base for vertical measurements
dof a perspective drawing: the line formed by the intersection of the ground plane and the picture plane
e: the area within which a baseball player must keep when running between bases; also: either of the lines on a baseball field that lead from home plate to first base and third base and are extended into the outfield as foul lines <It surprised me that I could buy a mass-produced home plate, although I don't know why it should have, considering that one can custom-order a baby nowadays. … I installed it carefully, securely, like a grave marker, then laid out a batter's box and . — W. P. Kinsella, Shoeless Joe, 1982>
f: a boundary line at either end of a court (as in tennis or basketball)
g: the lower horizontal guideline for aligning capitals in freehand lettering
h: a known quantity used as a control for further experimentation
: a usually initial set of critical observations or data used for comparison or a control
: a starting point <a kind of from which all discussions … must proceed — Nicholas Lemann>
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Medical Definition of BASELINE
: a set of critical observations or data used for comparison or a control