Tardiness is the quality of being late. When people don't show up on time, they're guilty of tardiness.
When you're late for something, you're tardy, so tardiness refers to the habit of being late. Because of their tardiness, some people almost never arrive at work, school, or other places when they're expected. Tardiness is very annoying for the people waiting for the tardy person, and too much tardiness at work could get you fired. Tardiness comes from the Latin word tardus, "slow, sluggish, dull, or stupid."