Spamdexing is the practice of keyword stuffing or otherwise manipulating an index for a website with the intention of increasing the website's ranking with search engines.
Techopedia explains Spamdexing
Spamdexing is technically a part of the process called search engine optimization (SEO), in which Web designers/developers build websites that are expected to rank high on search engine results page. However, the IT community has generally categorized spamdexing as part of "black hat SEO," which is the use of unethical methods to rank websites.
The methods used for spamdexing vary, including the creation of dummy content pages, keyword stuffing in meta-tags or otherwise promoting specific index results. Along with many other kinds of black hat SEO techniques, spamdexing has been a major source of frustration for dominant search engine operator Google as the company keeps creating new algorithms for rewarding honest sites and punishing unscrupulous ones. Some kinds of spamdexing and black hat SEO techniques have become generally obsolete as Google algorithms get updated and have changed the way that Internet sites are ranked.