A nitrocellulose slide (or nitrocellulose film slide) is a glass microscope slide that is coated with nitrocellulose that is used to bind biological material, often protein, for colorimetric and fluorescence detection assays.  For this purpose, a nitrocellulose slide is generally considered to be superior to glass, because it binds a great deal more protein, and protects the tertiary structure of the protein (and other biological material, i.e.: cells).  Typically, nitrocellulose slides have a thin, opaque film of nitrocellulose on a standard 25mm × 75 mm glass microscope slide.  The film is extremely sensitive to contact, and to foreign material; contact causes deformation and deposition of material, especially liquids.
A nitrocellulose slide is different from a nitrocellulose membrane, which usually filters protein from solution (i.e.: physician's office pregnancy tests), but that it serves a similar goal: to detect the presence and/or concentration level of certain biological material.