Centre line (CL) thrust is an absence of asymmetrical thrust from an aircraft. For example, a Boeing 757 has a wide distance between engine #1 and engine #2. If it were to sustain an engine failure in the #1 engine, the asymmetrical thrust would cause the plane to yaw (on the vertical axis) to the left. But on CL thrust airplanes this is not the case.
The following aircraft are all classified by the FAA as CL thrust aircraft:
References
Centre line thrust Wikipedia(Text) CC BY-SA