The Buddha-nature, the absolute, as eternally existent, i.e. the bhūta-tathatā.

A Dictionary of Chinese Buddhist Terms, William Edward Soothill and Lewis Hodous

Buddha-nature

(Sanskrit, buddhatā, buddha-dhātu; Jap., busshō). In early Buddhist thought, this term referred to the potentiality for becoming a Buddha through the traditional methods of study and religious practice. Discussion centred on clarifying what kind of beings had this potential and how it was to be developed. After the rise of Mahāyāna Buddhism, some thinkers began to question the validity of the distinction between practice and enlightenment (bodhi), or between potentiality and attainment, and so Buddha-nature gradually came to be seen not as a potential, but as the inherent Buddhahood of every sentient being. In this setting, the task then was not to achieve Buddhahood, but to uncover it. For example, the Platform Sūtra of the Sixth Patriarch compares Buddhahood to a full moon covered by clouds. When the clouds blow away, the moon is revealed. It does not gradually come into being nor is it constructed. Similarly, each being is already a Buddha, but this fact is obscured by defilements and impurities. Once these are removed, one's Buddhahood becomes manifest.

Still later, Chinese and Japanese Buddhism came to question the distinction between sentient and non-sentient beings. In this context, some schools came to assert that every phenomenon whatsoever has Buddha-nature, not just sentient beings. One of the goals of practice in this context is to see the unity of all things in the world based on the commonality of Buddha-nature.

Another strain of thought, dating back to Indian Mahāyāna sources, used the term Buddha-nature as a gloss for ultimate reality, or the final nature of all existents. In most Indian and Tibetan forms of Mahāyāna, Buddha-nature is generally understood as a synonym for emptiness (śūnyatā) and a lack of any abiding core of being (ātman). For example, Vasubandhu in his Treatise on Buddha-nature equated the term with both emptiness and nirvāṇa. However, under the influence of the Tathāgata-garbha teachings as developed in the Tibetan concepts of extrinsic emptiness, and parallel doctrines in Chinese Buddhism, Buddha-nature came to be seen as a more substantial presence endowed with positive attributes, often termed ‘ātman’ in such sūtras as the Mahāyāna Nirvāṇa Sūtra.

A Dictionary of Buddhism, Oxford University Press, 2003, 2004 (which is available in electronic version from answer.com)