a stain used to differentiate bacteria (typically mycobacteria). The Ziehl-Neelsen stain is a common acid-fast stain.

<technique> A staining technique used to determine the cell wall property of a microorganism. After stained with dye such as hot carbolfuschin, an acid-fast organism, (for example Mycobacterium species) will retain the colour in its cell wall after being washed with acid-alcohol. (13 Nov 1997)