A sentient being is a being who is subjectively aware; a being who has interests; that is, a being who prefers, desires, or wants. Those interests do not have to be anything like human interests. If a being has some kind of mind that can experience frustration or satisfaction of whatever interests that being has, then the being is sentient.
We engage in speciesist thinking when we claim that a being must have a humanlike mind to count morally. That is, it is speciesist to claim that a being must have a reflective sense of self awareness, or conceptual thought, or the general ability to experience life in the way that humans do in order to have the moral right not to be used as a resource. As long as there is someone there who is subjectively aware and who, in that being’s own way, cares about what happens to him or to her, that is all that is necessary to have the moral right not to be used as a resource.
Is there uncertainty as to where the line is between sentient and nonsentient? Of course there is. It is, however, clear beyond any doubt that all of the animals that we routinely exploit–the fish, cows, pigs, sheep, goats, chickens and other birds, lobsters, etc. are sentient. So we know everything we need to know to make the moral decision to stop eating, wearing, or using those animals. — AbolitionistApproach.com