The line at the back of the court, running parallel to the net. Each baseline is 36 feet wide (including doubles alleys). Most modern tennis is played from behind the baseline, with players trading groundstrokes until someone creates an opening. 'Playing the baseline' means staying back and hitting groundstrokes rather than coming to the net.
"Both players are camped out on the baseline, hitting heavy topspin forehands to each other. Twenty-shot rally."