For a physical stain (like on clothing or a surface):
* Tache: This is the most common and general term for a stain.
For a stain that leaves a mark or blemish:
* Tache: This word also works here.
* Tâché: This word is more formal and emphasizes the mark left by the stain.
For a stain that is a mark of dishonor or disgrace:
* Tache: This word can be used in this context as well.
* Souillure: This word specifically refers to a moral stain or a mark of shame.
For a stain that is a discoloration:
* Décoloration: This word refers to a change in color, often due to a stain.
Examples:
* Il y a une tache de vin rouge sur ma chemise. (There is a red wine stain on my shirt.)
* La guerre a laissé une tache indélébile sur l'histoire de ce pays. (The war left an indelible stain on the history of this country.)
* Cette peinture a une légère décoloration. (This painting has a slight discoloration.)
The best word to use will depend on the specific context.