Gain and phase matched cable
Two cables that are gain and phase matched have their complex attenuation, that is their gain from end to end in dB, or their loss if you want to call it that, and their phase shift in degrees, that is their 'electrical length', the same to within a specified tolerance of each other, up to a specified frequency.

So for instance, two 1m cables matched to 0.1dB and 1 degree over DC-10GHz would be very expensive, and may not even be obtainable at all. Relax the spec to 1dB and 10 degrees over DC to 1GHz, I could make them on my bench without specialist equipment.