Dirtiness is the state of being covered with dirt, grime, mud, or other unwanted substances. When your hands have dirtiness on them after playing outside, they're coated with soil and need washing. When a kitchen has a layer of dirtiness, it means grease, crumbs, and sticky spots have built up on the counters and floor.

The word describes both the actual mess and the quality of being unclean. A room's dirtiness might be obvious from muddy footprints on the carpet, or subtle, like dust gathering behind furniture. Scientists and doctors care deeply about dirtiness because germs and bacteria can thrive in dirty conditions. Before surgeries were performed in clean operating rooms, patients often died from infections caused by the dirtiness of medical tools and environments.

The opposite of dirtiness is cleanliness, which comes from regular washing, sweeping, and tidying up. A little dirtiness after hard work or outdoor play is natural and easily fixed, but letting dirtiness accumulate can lead to bigger problems that take much more effort to solve.