It could be default and everybody would understand it, although it's not completely correct, because once the user interacts with the button (for example, it's pressed), then s/he may miss which status was the default.
Also, some applications have buttons with different statuses on load (for example, ACTIVE status) , so for that button, the default would be active, which of course is not correct.
In short, the answer is as simple as NORMAL. See examples below:
From Material design
Other UI examples:
But again: default is understandable. You may also find this state named as idle, static or base as well (and probably in some other ways)