The USS Enterprise-D surrounded by two-dimensional lifeforms
A two-dimensional lifeform is unlike most typical lifeforms that exist in the galaxy.
Generally, lifeforms exist in three dimensions, but these such lifeforms are known to exist in a single plane only (and as such have either no length, no height, or no width). Some of these beings make their homes in cosmic strings, objects a single proton wide but with lengths measuring in kilometers or more. These strings have a gravitational attraction like black holes or homing beacons for two-dimensional beings.
Because they only exist in a single plane, starship sensors have difficulty detecting their existence, even through the use of Bayesian functions. One such example is when they surrounded the USS Enterprise-D in 2367. In that instance, Data ultimately detected their presence by using two probes to scan the space around the ship.
Because of their strange nature, two-dimensional lifeforms can interact in strange ways with starship engines. They exert forces similar to gravitational forces and can cause a ship's structural integrity field to begin to break down when the starship's warp drive is used to break free of their influence.
Because they can share emotions, large numbers of two-dimensional lifeforms gathered in one area can overwhelm Betazoid empathic and telepathic abilities, causing painful headaches. (TNG: "The Loss")