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