Cheating to get a webpage to the top of search results. The oldest and simplest trick is repeating a popular word all over the page — sometimes hidden, in white text on a white background, or crammed into a corner — so the search engine counts the word many times and concludes the page must be about it. That particular trick is called keyword stuffing, and spamdexing is the wider family it belongs to: also buying links, building whole fake sites that point at each other, or showing the search engine one page and the visitor a different one. The word is disapproving and always has been; nobody describes their own work this way. It is a niche, technical word, most common among people who work on websites, and it has faded somewhat because search engines got much better at catching these tricks — the honest version of the same effort is called SEO. Treat it as uncountable: 'spamdexing is against their rules', not 'a spamdexing'.