Sludge is a heavy, sticky substance that is found on certain planets and also created as the byproduct of certain creatures. Sludge usually appears as a thick, black substance, although other colors are possible. Sludge is somewhat similar to slime. Both substances are sometimes known in the vernacular as "goo".
Some creatures exist by living in sludge. The Bulgallian rat is one such animal. The creature Armus also appeared to have been made entirely out of sludge; however Armus was capable of manipulating his sludge to the point of even blocking communications signals. (TOS: "Coming of Age", TOS: "Skin of Evil")
Sludge can also be created artificially as a result of machinery malfunction. Miles O'Brien once found a pile of sludge in a power conduit of Deep Space 9, the result of that station’s aged and failing Cardassian systems. (DS9: "In the Hands of the Prophets") Components of several ship's systems on the USS Enterprise-D were turned into sludge after Nitrium parasites metabolized the nitrium found aboard the ship. (TNG: "Cost of Living")