Base Line Definition

noun
A line serving as a base.
Webster's New World
Any starting or reference point, figure, amount, level, etc. with which others can be measured or compared.
Webster's New World
A horizontal line measured with special accuracy to provide a base for survey by triangulation.
Webster's New World
The unmarked lane, six feet (1.8 m) wide, between any two consecutive bases: a base runner who does not stay within this lane may, in certain situations, be called out by an umpire.
Webster's New World

End line; esp., an area of the court just inside this line.

Webster's New World