Nebulas (meaning "cloud" in latin) are interstellar clouds consisting of various spacial debris such as: dust, ionized gasses (typically various gasses are present in a single nebula) and asteroids. Typically the various gasses react with one another sometimes resulting in intense electrical storms, various forms of radiation and other diverse effects such as natural dampening fields. Nebulas typically interfere with sensor readings, blinding ships travelling or caught within.
In 2366 the USS Enterprise-D hid within a nebula containing 82% dilithium hydroxyls, magnesium, chorium, attempting to elude the persuing Borg cube. The nebula disrupted the cubes sensors however later the cube adapted by firing magnetometric guided charges systematically in-to the nebula, ultimately driving the Enterprise to retreat.