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Quantum field theory (QFT) includes the concept of a false vacuum: in QFT, the term vacuum indicates the lowest possible state of energy, and a false vacuum is a state of energy that "seems" lowest (i.e., metastable) in that there is no lower state within the reach of normal interactions. (This technical definition of vacuum relates to the common definition because the emptiest possible space does include some quantum activity, but the least possible amount.)
If the false vacuum has the opportunity to interact with a true vacuum (the truly lowest state of excitation, the ground state), the false vacuum exhibits a negative pressure in relation to the true vacuum, causing the false vacuum to rapidly shrink. Some (cosmological) theories of inflation, including the original one, suppose it to be such an interaction involving a false vacuum present just after the Big Bang.
The term false vacuum is sometimes mentioned in the notion that the current universe might well be in a false vacuum and could undergo a false vacuum decay, which would end the universe as we know it.