This paper defines the research area of socially assistive robotics. While much attention has been paid to robots that provide assistance to a human through physical contact (which we call contact assistive robotics), and to robots that entertain through social interaction (social interactive robotics), there has not so far been a clear definition of socially assistive robotics. We summarize active social assistive research projects and classify them by target population, domain, and interaction methods. While distinguishing these from socially interactive robotics endeavors, we discuss challenges and opportunities that are specific to socially assistive robotics as a growing field.