A panhard rod is a bar (usually steel) used in automobile suspension systems to stop the rear axle of a car from moving horizontally during cornering.
Panhard rods are usually attached to the frame of a car at one end, and the rear axle housing at the other.
If spring rates or ride height have been been changed on a vehicle, the panhard rod will usually need to be replaced with an adjustable equivalent, or else the rear track will be permanently offset to one side, causing 'crabbing'.
Also known as a track rod.
An additional note for the linguistically inclined: the word panhard is properly pronounced 'pannard' but most people pronounce it as it's written. If in any doubt, I recommend you say 'track rod' and if they look at you funny, explain what you mean, then smile knowingly when they say 'oh, you mean a pan-hard rod!'.