The quantum vacuum is the lowest-energy state of a quantum field, containing no real particles but still having quantum fluctuations.
In quantum field theory, particles are excitations of underlying fields. Removing all detectable particles does not make every field observable exactly zero, because field components obey uncertainty relations. The vacuum therefore has correlations and fluctuations, even though it has no real particles present in the ordinary particle-counting sense.
These properties have measurable consequences. Vacuum fluctuations contribute to effects such as the Casimir force and the Lamb shift, although the detailed interpretation depends on the theory and observable. The vacuum can also be modified by boundaries, media, acceleration, or curved spacetime.
The quantum vacuum should not be confused with a classical empty container or with the obsolete picture of a literal sea of negative-energy electrons. Virtual particles are useful terms in perturbative calculations, but they are not ordinary particles that can be detected individually in the vacuum.