When my kids were quite young and full of all those questions parents get asked, somehow the questions got around to death and cold and other hard to define things.
I tried to explain that many terms are defined as the absence of some quality that their opposite terms have. As in “death” is the absence of “life.” Or that “cold” is the absence of “heat.”
We batted around a few other such concepts and that how to define one of the pair you really only needed to define one of them and then use the “absence of” idea to define the other.
One of the kids came up with the thread title and it struck me then and strikes me now as as legitimate a pair of definitions as those more heady ones we’d been discussing.
On a philosophical note, would you define things as <thing> is the absence of <normal condition>? The reason I ask this is because I found myself thinking that perhaps some of your definitions are / could be backward, such as:
Dark being the absence of light (rather that the other way around),
Sick being AO well
Sad being AO happy