The Mutara Nebula
The Mutara Nebula is an interstellar dust cloud located in the Mutara Sector. The nebula contains high levels of static discharge and comprises largely of ionized gases. The combined effect of this makes a starship's sensors highly unreliable and shields inoperable when inside.
In 2285, it was the site of the horrific Battle of the Mutara Nebula. The battle ended when Khan Noonien Singh detonated the experimental Genesis Device inside the nebula, causing matter within it to re-form into the Genesis Planet. (Star Trek II: The Wrath of Khan)
Background
It is unclear if the entire nebula was consumed in the detonation (images from the USS Enterprise's escape suggest part of it survived). Also unclear is how the Mutara Nebula relates to a Mutara class nebula. Pursuant to this question, in the Voyager episode "One", the nebula being transited while the crew were in stasis was referred to by Seven of Nine as "the Mutara Nebula."
Visuals of the Mutara Nebula, turned upside-down and slightly modified, were reused as the Chamra Vortex in DS9: "Vortex". Visuals of the nebula also appear in modified form in certain shots of the Paulson Nebula in TNG: "The Best of Both Worlds".
As for the name of the nebula, it is possible that Mutara comes from the Latin verb Mutarē, meaning "to change". This would be consistent with genesis meaning "to create."