The part of an advertising message/phrase that encourages a user to take an action. (i.e. “sign-up”, “buy now”, “call now”, “subscribe”, “learn more”, and “install now”)
Regarding to website, a call-to-action, in the context of the empowered marketer, refers to any link/text that encourages site visitors to visit your landing page and engage in an action like filling out a form to get a premium offer such as a free ebook, white paper, etc.
Every successful business blog post needs some sort of call-to-action to help convert readers into leads. An example of a call-to-action would be the banner image on this very page!