Words in theoretical physics have gotten cheap. The word definition has moved from its mathematical dependence, that existed up until the 1960's or so, to layperson type guesses, opinions, or escapism when physicists' refuse to acknowledge that which they do not know about the foundations of physics.
The circumstance that prompts this message has to do with the word 'proof'. Rather than admit that their support for their theories is insufficient and even fundamentally artificial, physicists reach out to mathematicians for help. While physicists, especially in textbooks, throw around answers, some of which lack empirical support, for what things are, when confronted by requests to take their stand formally, they excuse themselves by citing that proofs belong to mathematics and do not apply to physics. Of course they don't.
Mathematics is the study of shortcuts for counting. Their proofs consist of whether or not their shortcuts work to reproduce the intended count or counts. Physics uses empirical evidence that occurs in the real world of Earth and Sky. Rather than admit even that their feet are on firmly on the Earth, physicists, in their effort to avoid answering questions such as: What is mass?, escape from the real world of relying upon repeatable empirical evidence and cry out that mathematics is the place to go for proofs.
It is ironic that, in the largest part, theorists argue that empirically unsupportable theories such as Relativity Theory are fact, while refusing to go on the record that they have proof for what they say.
In physics, the word 'proof' does not apply to whether or not a dropped stone will fall down. Physicists would rather allow for the eventual stone that falls up rather than have to admit that they lack physics proofs for so much of, even in the fundamentals of, theoretical physics.
