An acid-fast dye is a type of laboratory stain used to identify bacteria that have a waxy layer in their cell walls, making them resistant to traditional staining methods. These dyes, such as Ziehl-Neelsen or Kinyoun stain, are able to penetrate the waxy layer and stain the bacteria, allowing for their identification under a microscope. The acid-fast stain is commonly used to diagnose diseases caused by acid-fast bacteria, such as tuberculosis and leprosy.