SEO- Search Engine Optimization
Definition: The term SEO mean Search Engine Optimization. This is not about optimizing the search engines, though. It is about optimizing websites for search engines. But why one needs to optimize a website?
To answer this question we need to understand what a search engine is.
Search engines as the way to find an info on the web appeared in the middle of 90's. They crawl the websites and index them in the database marking them as having one or another keyword in its content. Thus, when someone put some query in the search-box of search engine, it quickly searched its database and found which indexed pages corresponded to that query.
Search engines has introduced SEO to get the more relevant information or results according to client's search terms.
Top 20 Search engines can be available here.
So, SEO is something that helps your site rank better in search engines.