The definitions are specific.
(They are defined in FAR Part 36).
I'm writing also the Take-off and approach noise definitions just for your information.
Sideline noise: is defined and measured 1476 ft (450 m) from the runway centerline at a point where the noise level after liftoff is greatest.
Take-off noise: is defined as the noise measured at a distance of 21,325 ft (6500 m) from the start of the take-off roll,
directly under the airplane.
Approach noise: is also defined and measured under the airplane when it is at a distance of 6562 ft (2000 m) from the runway threshold.
For each of these conditions the maximum noise level is a function of maximum takeoff gross weight, and for the take-off case the limits depend also on the number of engines.