A warp bubble is a subspace phenomenon, a disruption in normal space with subspace properties that is based on the principle used to create a warp field. While a warp field radiates and dissipates, a warp bubble can hold its shape, but eventually becomes unstable and collapses inward.
In 2366, Wesley Crusher conducted an experiment generating a static warp bubble, which accidentally trapped his mother, Dr. Beverly Crusher, in an alternate dimension of her own inadvertent creation. (TNG episode: "Remember Me")
In 2377, after the USS da Vinci had been transported to an alternate dimension by Rod Portlyn, chief engineer Nancy Conlon proposed using a static warp bubble in a similar way to return to their dimension. This idea was ultimately deemed impractical, as there was no way to control the dimension they would travel to. (CoE eBook: The Art of the Comeback)