Definition:

It is the portion of a marketing broadcast that aims to influence an individual to execute a favorable action.

Information:

A call to action strives to convince an individual to execute a specific action promptly. “Buy Now!” and “Register Today!” are some general instances. It aims to enhance the market’s acknowledgment rate to the ad copy, as its inadequacy can cause an individual to disregard the ad and advance to another matter.

Helpful Articles Related to Call to Action (CTA):

Call to Action Buttons Best Practices Guide
UX Movement (February 21, 2011)

Call to Action Buttons: Examples and Best Practices
Smashing Magazine (October 13, 2009)

10 Techniques for an Effective ‘Call to Action’
Boag World (January 22, 2009)