In Buddhism, bodhicitta[1] (Ch. 菩提心, pudixin, Jp. bodaishin, Tibetan jang chub sem, Mongolian бодь сэтгэл) is the wish to attain complete enlightenment (that is, Buddhahood) in order to be of benefit to all sentient beings -- beings who are trapped in cyclic existence (samsāra) and have not yet reached Buddhahood. One who has bodhicitta as the primary motivation for all of their activities is called a bodhisattva.

Etymologically, the word is a combination of the Sanskrit words bodhi and citta. Bodhi means 'awakening', or 'enlightenment'. Citta may be translated as 'mind' or 'spirit'. Bodhicitta can therefore be translated as 'mind of enlightenment' or 'spirit of awakening'.

Bodhicitta may also be defined as the 'Union of Compassion and Wisdom'. While the Compassion and Wisdom aspects of Bodhicitta are actually highly dependent on each other, in the Mahāyanā tradition they are often referred to as:

Relative Bodhicitta, in which the practitioner works for the good of all beings as if it were his own.
Absolute, or ultimate, Bodhicitta, which refers to the wisdom of shunyata (śunyatā, a Sanskrit term often translated as 'emptiness', though the alternatives 'openness' or 'spaciousness' probably convey the idea better to Westerners). The concept of "śunyatā" in Buddhist thought does not refer simply to nothingness, but refers, loosely, to freedom from attachments (particularly attachment to the idea of a static "self") and fixed ideas about the world and how it should be. The classic text on śunyatā is the Prajñāpāramitā Hrdaya Sūtra, a discourse of the Buddha commonly referred to as the "Heart Sūtra."
So, the term bodhicitta in its most complete sense would combine both:

the arising of spontaneous and limitless compassion for all sentient beings, and
the falling away of the attachment to the illusion of an inherently existent "self."
Some bodhicitta practices emphasize the absolute (e.g. vipaśyanā); others emphasize the relative (e.g. metta), but both aspects are essential to develop on the path to enlightenment. The Relative without the Absolute can degenerate into pity and sentimentality while the Absolute without the Relative can lead to nihilism and lack of desire to engage other sentient beings for their benefit. The cultivation of both the relative and absolute aspects of Bodhicitta is an important part of all Mahāyāna practices, including in particular the Tibetan Mind Training practices of tonglen and lojong.

Bodhicitta may be viewed as having different levels: one useful classification is that given by Patrul Rinpoche in his Words of My Perfect Teacher. He states that the lowest level is the way of the King, who primarily seeks his own benefit but who recognizes that his benefit depends crucially on that of his kingdom and his subjects. The middle level is the path of the boatman, who ferries his passengers across the river and simultaneously, of course, ferries himself as well. The highest level is that of the shepherd, who makes sure that all his sheep arrive safely ahead of him and places their welfare above his own