Counterstain is a term used to describe the process of staining a sample or tissue with a contrasting dye after the primary stain has been applied. This technique is used to highlight specific structures or cells, and can improve contrast and clarity in microscopic images. Some synonyms for counterstain include contrast staining, differential staining, secondary staining, and double staining. Contrast staining refers to the use of a contrasting dye to enhance the contrast between different structures or cells in a sample. Differential staining involves using different stains to highlight different structures in a sample, while secondary staining and double staining are both used to describe the application of a second stain after the primary stain has been applied.