Definitions for line drive
Random House Webster's College Dictionary
a batted baseball that travels low, fast, and straight.
Ref: Also called liner. 1 3
Origin of line drive:
liner, line drive(noun)
(baseball) a hit that flies straight out from the batter
"the batter hit a liner to the shortstop"
A batted ball hit hard enough and low enough that it appears to travel in a relatively straight line.