"Though we commonly fail to distinguish sentience from consciousness, I think it makes sense to use the term sentience to refer to a more generic property of organisms with brains -- their spontaneous adaptive, self-versus-nonself organization -- and to use the term consciousness to refer to the way they represent aspects of the world to themselves." (Deacon, The Symbolic Species, p.455)
Peter Singer uses sentience as a shorthand for the capacity to suffer or to experience enjoyment or happiness. Singer argues that any sentient being has interests and hence is a subject for ethics.