Most online marketing tactics and strategies in general are considered mainstream as the internet has matured from being a geek haven to a geek haven mixed with all levels of regular folks and business professionals.
Now, the term itself 'SEO' is definitely becoming more mainstream, but I wouldn't say it's mainstream as of now. When I talk to most people on the street, random individuals with jobs like insurance sales, customer service, banker, apartment manager etc, the majority do not associate SEO with the term Search Engine Optimization and even when presented with the full term, they only have a vague understanding of what that could possibly mean, which they interpreted incorrectly from the words themselves.
Common real responses include (get ready for a chuckle):
Is that dong something with search engines? Optimization sounds complicated are you a programmer? Sound technical did you have to go to school for that? What do you mean optimization, are you a spammer?











