The answer is balance. Milady defines it in almost the exact words the question uses: razor balance refers to the weight and length of the blade relative to that of the handle.
The book describes a quick way to test it: open the razor and rest it on the index finger at the pivot — if the head moves up or down, the razor isn't well balanced. Temper, grind, and size are all separate razor-quality factors covered in the same section, each with its own distinct definition unrelated to weight distribution.
Milady Standard Barbering, 6th ed., Ch. 5 (Implements, Tools, and Equipment), p. 121: "Razor balance refers to the weight and length of the blade relative to that of the handle. A straight razor is properly balanced when the weight of the blade and handle are equal."
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