It's a wave definition in where you find a 'imbalance' of 'forces'. Those create a 'suction' between the plates. 'Zero point 'energy' * And no, it's not the same as negative energy, because that one is.locked to gravity
Apart from the following on the Wikipedia page on the Casimir effect. "The Casimir effect can be understood by the idea that the presence of conducting metals and dielectrics alters the vacuum expectation value of the energy of the second quantized electromagnetic field. Since the value of this energy depends on the shapes and positions of the conductors and dielectrics, the Casimir effect manifests itself as a force between such objects." https://en.m.wikipedia.org/wiki/Casimir_effect So what is the second quantised electromagnetic field?