In this post, you will read the definition of SEO by different sites. You can co-relate between them.
Search engine optimization is the process of affecting the online visibility of a website or a web page in a web search engine’s unpaid results—often referred to as “natural”, “organic”, or “earned” results. — According to Wikipedia.
Basically, Search Engine Optimization refers to the collection of techniques and practices that allow a site to get more traffic from search engines (Google, Yahoo, Microsoft). SEO can be divided into two main areas: off-page SEO (work that takes place separate from the website) and on-page SEO (website changes to make your website rank better). This tutorial will cover both areas in detail! Remember, a website is not fully
optimized for search engines unless it employs both on and off-page SEO.
Search engine optimization (SEO) is the practice of increasing the quantity and quality of traffic to your website through organic search engine results. — According to Moz.com
Search engine optimization is the process of improving the visibility of a website on organic (‘natural’ or un-paid) search engine result pages (SERPs). — According to whatisseo.com
You can see in the above definitions that all are want to say that SEO is a process by which you can improve your website or blog visibility on Search Engines like Google, Bing,Yahoo etc or it is a process by which you can gain more traffic to your website by means of Search Engines naturally or Organically (without spending money).