“foil” in this context

at the end of dorian gray, this dialogue occurs between lord henry and the duchess

He glanced about as if in search of something. 'What are you looking for?' she inquired. "The button from your foil,' he answered. 'You have dropped it.' She laughed. 'I have still the mask.' 'It makes your eyes lovelier,' was his reply. She laughed again. Her teeth showed like white seeds in scarlet fruit.

what does foil mean in this context? none of the definitions i looked up make sense…