Wordnet defines robot as "a mechanism that can move automatically". It defines robotics as "the area of AI concerned with the practical use of robots".
From this information, I would define a robot as "a machine that can perform physical tasks automatically". I would define robotics as "the practical use and study of robots".
Referencing
Ware P, Cooke P, Opfer D, Wall J. (2005), Information and Software Technology 7-10, Jacaranda: Milton, QLD
Wordnet, Cognitive Science Laboratory: Princeton University retrieved 26 March 2007 from http://wordnet.princeton.edu/perl/webwn?s=robotics