Even in the absence of matter, there’s still the quantum foam. The most perfect vacuum is teeming with this energy. To truly understand the nature of ‘nothing,’ one would need to venture beyond spacetime itself—and even then, it’s not guaranteed that ‘nothing’ would be found. Physics suggests that anything existing outside of or predating spacetime would generally have no impact on us; it doesn’t necessarily explain what the ‘outside’ might be like.
I’d say nothing is less a definition but rather an informal shorthand for how we percieve at macro scale with our wrinkly 4D brains.