Invisible/hidden text is any text that search engines can view, but readers can’t. Usually, black hat SEO uses hidden text to incorporate extra or irrelevant keywords to boost the page’s rankings across more search terms. There are a few different methods of incorporating hidden text that are against search engine guidelines. If you were to set the font size to zero, make the text white on a white background, use CSS to make the text appear off-screen, or hide a link by only attaching it to a single character (e.g., a period), those would all be black hat examples of hidden text.