Unambiguously referring to "spiciness"
Spiciness is a taste perception, and as such, it is simply subjective. There is no way to create an objective scale for rating it. 

I read your comment about "objective spiciness", but it is not something that can exist. It is based on the erroneous assumption that the spiciness you perceive is a 1:1 measurement of some quality of the food. This is not true. Even foods with the same amount of capsaicin will taste differently spicy to the same person, or the same food can taste differently spicy to the same person in two separate meals. So no, all scales will be subjective, with all the usual consequences of subjective rating scales.