Filth is disgusting dirt or waste that makes something extremely dirty or unclean. When a garbage can hasn't been emptied for weeks and food has rotted inside, that sticky, smelly mess is filth. When a bathtub gets covered in grimy soap scum and mold, that's filth too.

The word suggests more than ordinary dirt: it means something truly revolting that makes you want to look away or hold your nose. You might describe your muddy shoes as dirty, but you'd call a trash-filled alley filthy or say it's covered in filth. The difference is the level of disgust involved.

The word filthy works as an adjective: a filthy room is covered in filth, and filthy language is full of crude words. People sometimes use filthy rich to describe someone with a huge amount of wealth, just to emphasize how extremely rich they are.