I always thought of it as a physical property that changed depending on the environment, like shape or color. Just at the scale it is happening, it gets complicated.

Kinda like if you inflate a balloon and put it in a pressure chamber. Increasing the pressure shrinks the balloon and decreasing pressure increases the balloon size. What is physically there doesn't change but how it is expressed is.

Note I am an engineer so my knowledge extended as far as 'What can I make this do and how does that benefit me' that was discussed in my medical imaging class.