The acronym “SEO” means Search Engine Optimization.
It is definitely impossible to give it a standard definition, as the SEO includes many aspects that need to be considered when preparing its strategy.
Theoretically, SEO is the combination of techniques and strategies that makes a website able to be found on the several Search Engines while users look for specific information on determined contents, products or services, without the support of paid advertisement.
SEO includes all those type of activities (both strategical and operational) that make websites visible through the results from the search engines to determine keywords, in order to be found from those users who are interested in that specific content.
In a few words, SEO is an essential marketing tool that allows to reach all those users interested in a certain type of argument/product/service and to attract them towards a specific website in the exact moment they request those kind of information from the web.
We can think of the SEO as a giant magnet able to attract towards one website all the targeted users through their search types.