What is SEO? An advanced definition.
SEO is a vast system which includes a wide range of services, techniques, tools, theories, platforms and pitfalls all based around the common goal of ranking a website first (or as high as possible) in the organic Search Engine Results Pages (SERPs) of such Search Engines as Google, Yahoo, Bing, Yandex and others.
SEO is not just modifying a website to rank higher in the results but is primarily based on tracking and understanding the Search Engine algorithms and attempting to reverse engineer them to divine the reasons that Search Engines rank pages the way they do.
Although the primary goal of SEO is the placement of your website higher in the organic results; such placement can be, in part, based on elements outside of your website and of your control. Such things as backlinks (links from other websites to yours) and even mere mentions of your brand, website's URL or the quantity of people sharing your content and commenting about your company, website or services, can affect your organic rankings.
Search Engines utilize these "signals" as a large part of their ranking methods which are primarily based on a set of computational rules called algorithms. Any Search Engine can have a multitude of algorithms which manage certain aspects (such as those listed above) that all affect your rankings in the organic results.
Key SEO ranking factors include (but are certainly not limited to):
Freshness of Content
The primary factor in SEO is relevance. Relevance is a mix of relation and usage. In language relevance is established by the usage of a term to describe or connect your brand in a relational sense. This means that mere usage defines relevance by creating such a relationship. For example; if a new brand is established called "XYZ123" - it could be applied to anything. You have no idea what that is until it is defined by adding words which create a relationship and hence a definition of that brand. XYZ123 could be a company which makes hats or a new mobile application for tracking your favorite sports team. It could be a new accounting service or a record label. By associating terms with the brand we define it. We make XYZ123 relevant to a subject by usage of terms that define that subject. Usage can also mean the quantity of times it is used and the quantity of people who use it. This relates to both popularity and authority. Popularity refers to overall usage by a group of people. Authority refers to usage by people who are considered to be authorities in that subject. Mere popularity does not constitute authority however authority includes an element of popularity.
Google's original PageRank algorithm was based on the use of citations as in academic papers. If enough people cite a work in a specific context then that work is deemed to be authoritative on the subject matter. The greater the amount of citations and the greater amount of the authority of those citing; then the greater amount of authority of the work (and author) being cited.
Backlinks are used as such citations as a website which links to another is considered to be casting a vote in favor of the relevance of that linked website to its own or to the topic which it is presenting. Over time people began to game the system by purchasing or creating such backlinks in order to rank higher. Google launched an algorithm named Penguin which greatly reduced people's ability to game the system based on backlinks. Although this did not cure the problem it did make things much more difficult to this type of person called a Link Spammer.
Relevance is however the most important element of SEO as Search Engines provide results based on the words which are searched, called queries or questions. Every term you enter in to a search is interpreted by the engine as a question. The answer is the results which are presented once you click "search". Originally such searches were based on the keywords used however Search Engines, especially Google, have created algorithms to attempt to ascertain the intent of the user and the topical context of the query. Google launched a new algorithm named "Hummingbird" which attempts to do this.
SEO is also defined as making your website relevant to the topic which you are targeting. Any topic includes certain words and phrases whether people, places or things, which are commonly associated with that topic. It is the goal of SEOs to define the topic and to define what terms, phrases, questions, locations, times and other data are applicable and relevant to the topic and to implement these into the content of the website. Hummingbird also looks at the contextual specificity of the query; in other words, what do you specifically mean by it. If you search for "hummingbird" do you mean the bird or the Google algorithm? More specifically you could be searching for tips on optimizing for Google Hummingbird algorithm or learning how to build a hummingbird feeder.
Optimization of content on a contextual basis means that you must define the specifics of the topic and seek out the terminology used to describe it and those words and phrases which are most commonly associated with it.
When using off-site SEO it is required to find the sources that are specifically related/relevant to the topic and to connect with them.
SEO is more than just putting keywords into your website or building links or gaining popularity; it is about building relevance and conveying that relevance to the Search Engine.