SEO (search engine optimisation) is the work carried out on a
website that ensures, new or old, that it can be found easily and ranks
successfully (in SERPs - search engine result pages) when a keyword
search is carried out in any of the major search engines: Google, Bing,
Yahoo!, Ask &c.
Being found easily in the organic SERPs means the website must rank
well for its particular subject or niche, meaning it must have
developed a certain amount of authority. To rank well and gain
authority for a particular subject your website will need to have had
some good quality SEO and keyword research carried out, this is
essential for good website optimisation.