Search Engine Optimization. SEObook.com, one of the foremost authorities on SEO defines it like this
Search engine optimization is the art and science of publishing information and marketing it in a manner that helps search engines understand your information is relevant to relevant search queries.
Wikipedia has its own definition.
I define SEO as the task of increasing your visibility in the search engines. Your company, your brand, or even yourself has a visibility in the search engines. This includes, primarily, your website, but other things effect visibility too. For instance, social media profiles will increase that visibility.
Take a look at this image.
This is the front page of the results for “todd e jones” in Google.
You will see that my website, toddejones.net, is at the top of the rankings which I am thankful for since the sites listed just below mine have been around a lot longer. However, if you look at the whole page you will see three social media profiles giving me a visibility of four different results on the front page.
I have circled the results. There is my Facebook page, my twitter profile and finally, my Linkedin profile.
As a result, you will see, that a simple thing like having social media profiles will increase your visibility online and ultimately your SEO for your website.