base line

noun

1. 1. a line serving as a base
   2. any starting or reference point, figure, amount, level, etc. with which others can be measured or compared
   3. Surveying a horizontal line measured with special accuracy to provide a base for survey by triangulation
2. ☆ Baseball the unmarked lane, six feet (1.8 m) wide, between any two consecutive bases: a base runner must normally stay within this lane
3. Basketball end line; esp., an area of the court just inside this line
4. Tennis the line at the back at either end of the court