It depends on who is invoking the term, really. But you're probably talking about a spacetime deformation of some kind. Insofar as we're currently aware, that means gravity, and possibly a lot of it.
Gravity warps space in two ways. First, it has an effect on particles in motion, commonly referred to as a "gravity well." Particles will be deflected from their vector by interaction with the gravity field. It's like a marble rolling across a tabletop. If the table is warped, the marble shifts direction. Second, we know that gravity affects the passage of time. A hard enough gravity well, such as might be found around massive black holes, could have potentially incredible effects on spacetime, or at least the math would suggest it.
Either way, fundamentally speaking, you're talking about a topological defect in an idealized "flat" plane of space. The catch is that it's pretty hard to find "flat" space. Only in the large voids that exist in a few areas of the sky where even galaxies are very wide-spread might you find flat space. Otherwise, you're in an area of space that's being deformed by gravity from any of a number of sources, the most easily-forgotten being the gravitational pull of a galactic center. Everything in the Milky Way is swirling around that dent. We forget about it because it's on a scale that's pretty much beyond our scope of comprehension, but it's there.