Sludge is thick, muddy, often disgusting semi-solid material that settles at the bottom of liquids or results from industrial processes. Picture the gunky muck at the bottom of a pond after it's been drained, or the grimy buildup in old pipes. That's sludge: too thick to pour easily, too wet to be solid, and usually pretty gross.

The word often describes waste products. Sewage treatment plants process tons of sludge every day, separating clean water from the thick, contaminated residue. Factories create industrial sludge as a byproduct of manufacturing. Even your car's engine can develop sludge if old oil breaks down and turns into a thick, sticky mess inside.

Sludge isn't always literal. People use the word to describe anything that slows things down or makes progress difficult. When a company has too many boring forms and unnecessary procedures, someone might complain about “bureaucratic sludge” gumming up the works. A research paper filled with confusing jargon might be criticized as “verbal sludge” that obscures the ideas.

The word captures that quality of being stuck, slow, and unpleasant. Whether it's actual muck or metaphorical obstacles, sludge is something you usually want to get rid of.