Subspace is an alternate dimension referred to in many science fiction settings. While the details of subspace vary, a common characteristic is that subspace phenomena and technologies can defy the known laws of physics by exceeding the speed of light.
Star Trek Subspace
In Star Trek, subspace lies behind many futuristic technologies. "Subspace fields" allow ships to travel at faster-than-light speeds, for instance. Messages transmitted through subspace also travel faster-than-light. Mass lightening is another subspace technology that somehow reduces the inertial mass of a starship, allowing it to accelerate quickly with a small amount of thrust for its true mass.
Normally, subspace only influences objects in normal space within the confines of a technologically created subspace field. The separation between normal space and subspace can thin, however, where intense subspace fields are used regularly. For example, constant warp-speed starship traffic through the Hekaras Corridor thinned the barrier between normal space and subspace, and the subsequent self-destruction of a starship's warp drive in the corridor created a rift between the dimensions. Direct exposure to subspace is extremely hazardous for Federation starships and presumably all other Star Trek vessels.
According to Star Trek engineers, subspace has multiple layers or "domains". At least one subspace domain is inhabited by technologically advanced aliens.
Limitations and Vulnerabilities
Ships that generate powerful subspace fields, such as starships traveling at warp speed, are highly visible on subspace sensors. A ship that might go unnoticed at sublight speed can be spotted from light-years away if traveling at warp speed. Not even a cloaking device can fully conceal a ship's subspace field.
Subspace technology can sometimes be a liability. For instance, the subspace shockwave from the Praxis explosion tossed the USS Excelsior like a leaf on the wind, even though the explosion occurred light-years away, meaning the wave should have had negligible force and energy due to the inverse-square law. Such a blast would have had little effect on a ship that wasn't so dependent on subspace fields. Similarly, "isolytic subspace weapons" create rifts into subspace that are attracted to the subspace fields created by warp drives.