An alternate realm made of organic fluid instead of empty space, home of Species 8472, whose rifts into our galaxy led to war with the Borg.
Fluidic space is a separate realm in which there is no empty vacuum; instead, the whole area is filled with a thick organic fluid. It is the home of Species 8472, powerful beings who live and travel within it in organic ships. It can be reached only through rare openings in the fabric of space, such as quantum singularities.
The Borg forced open a gateway into fluidic space in the hope of assimilating Species 8472, but the move went badly wrong. The new species proved immune to assimilation and launched a devastating counterattack into the Delta Quadrant in 2373. Captain Janeway made an alliance with the Borg, offering a weapon based on modified Borg nanoprobes in exchange for safe passage. Voyager later entered fluidic space itself to strike at 8472, and Seven of Nine helped close the rift after the alien forces were driven back.
Fluidic space showed that even the Borg could be overpowered. It brought Seven of Nine into the Voyager crew, and its war marked a turning point for the series and for the Borg's place in the galaxy.