For many years after the introduction of the concept of search engine optimization, keywords and back links became the most important factor in search engine marketing. After Google's AdSense came to picture, the frequency of duplicate content took a rise. Websites were created with no or least importance with template after template filling up the website with keywords so that it could attain a ranking position.

This was when the experts in search engine marketing realized the importance of introducing a technique which would cut down on the number of duplicate pages and would help in creating a useful and well constructed website. The solution to their problems came in the face of LSI or Latent Semantic Indexing.

Latent Semantic Indexing was used initially by AdSense to make sure that the adverts that fit a theme of a webpage appear on the page. The LSI algorithms check the words in the web page and hence determine the concept or theme of the webpage. The basic concept of LSI in search engine marketing involves the analysis of words, the use of synonyms and related terms which can study the theme of the webpage. This technique is not used as a replacement to the old search engine marketing technique of keyword analysis; it only compliments it and further increases the probability of better search results.

One drawback of the LSI system is that it is not always accurate. The fact that LSI is based on mathematical calculations of rules and algorithms makes it less accurate than keyword analysis which is based on simple text matching fundamental.

The term Latent Semantic Indexing can be explained in simple words considering the literal meaning of each word. The dictionary meaning of the word 'Latent' is something which is present but not visible, in Latent Semantic Indexing the term means that a particular word say 'press' will have its meaning hidden until it is supported by another set of words. Semantic is the meaning of the term that has been used in the text considering the supporting texts in the sentence. For example 'press conference' or 'press the doorbell.' Indexing on the other hand is identifying the meaning of the word or language that is being used in context to the words.

The introduction of LSI in Search Engine Marketing has made it essential to use not only specific keywords but also related words so that the overall theme of the page is defined well.