Sentient beings is a technical term in Buddhist discourse. Broadly speaking, it denotes beings with consciousness or sentience or, in some contexts, life itself. Specifically, it denotes the presence of the five aggregates, or skandhas.
While distinctions in usage and potential subdivisions or classes of sentient beings vary from one school, teacher, or thinker to another—and there is debate within some Buddhist schools as to what exactly constitutes sentience and how it is to be recognized—it principally refers to beings in contrast with buddhahood. That is, sentient beings are characteristically not enlightened, and are thus confined to the death, rebirth, and suffering characteristic of Saṃsāra. However, Mahayana Buddhism simultaneously teaches (in the Tathagatagarbha doctrine particularly) that sentient beings also contain Buddha-nature—the intrinsic potential to transcend the conditions of samsara and attain enlightenment, thereby becoming a Buddha.
In Mahayana Buddhism, it is to sentient beings that the Bodhisattva vow of compassion is pledged. Furthermore, and particularly in Tibetan Buddhism and Japanese Buddhism, all beings (including plant life and even inanimate objects or entities considered "spiritual" or "metaphysical" by conventional Western thought) are or may be considered sentient beings. Getz (2004: p.760) provides a generalist Western Buddhist encyclopedic definition: