A watermark in the context of authentication is a recognizable image or pattern embedded into a document or digital file. It serves as a security feature to verify the authenticity of the content, making it difficult to replicate or alter without detection. Watermarks can be visible, like a logo, or invisible, embedded within the file's data. Watermarks are commonly used in various fields, including photography, publishing, and banking. They help protect intellectual property and prevent forgery by ensuring that the original source can be identified. This technique is essential for maintaining trust in digital and printed materials.