A stain is a mark or discoloration that's hard to remove, usually caused when something like juice, mud, ink, or food soaks into fabric, wood, or another surface. When you spill grape juice on a white shirt, it leaves a purple stain that won't simply wipe away because the liquid has penetrated the fabric's fibers.

Stains are frustrating because they're stubborn. A little dirt might brush off easily, but a stain sets in and requires serious cleaning effort. Different stains need different treatments: chocolate stains, grass stains, and grease stains each respond to specific cleaning methods. Sometimes an old stain becomes permanent, which is why people often say “that will stain” as a warning to be careful.

The word can also mean to deliberately color wood or other materials. When someone stains a wooden deck, they're applying a colored treatment that soaks in to protect the wood and make it look richer. This kind of stain is intentional and desirable, unlike the accidental kind.

In a figurative sense, something that damages a person's reputation can be called a stain on their character or record. When someone lies or cheats, that behavior can stain their reputation, making others see them differently, just as a literal stain changes how cloth looks.