In Buddhism, one who has gained insight into the true nature of existence, has achieved nirvana, and will not be reborn. Theravada Buddhism regards becoming an arhat as the goal of spiritual progress. It holds that a seeker must pass through three earlier stages before being reborn in a heaven as an arhat. Mahayana Buddhism criticizes the goal of becoming an arhat as selfish and considers the bodhisattva to be a higher goal because the bodhisattva remains in the cycle of rebirths to work for the good of others. This divergence of opinion is one of the fundamental differences between Theravada and Mahayana Buddhism.
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The word "arahan" literally means "worthy one (an alternative folk etymology is "foe-destroyer" or "vanquisher of enemies) and constitutes the highest grade of noble person—ariya-puggala—described by the Buddha as recorded in the Pali canon. The word was used (as it is today in the liturgy of Theravada Buddhism) as an epithet of the Buddha himself as well as of his enlightened disciples. The most widely recited liturgical reference is perhaps the homage: Namo Tassa Bhagavato, Arahato, Samma-sammbuddhassa.—Homage to him, the Blessed One, the Worthy One, the perfectly enlightened Buddha.
In Theravada, although the Arahants has achieved the same goals which is identical to Buddha, there are some differences among Arahant due to the way of their practice.
Mahayana Buddhists see the Buddha himself as the ideal towards which one should aim in one's spiritual aspirations. Hence the arhat as enlightened disciple of the Buddha is not regarded as the goal as much as is the bodhisattva. Bodhisattva carries a different meaning in Mahayana Buddhism compared to Theravada Buddhism. In the Pali scriptures the Tathagata when relating his own experiences of self-development uses a stock phrase "when I was an unenlightened bodhisattva". Bodhisattva thus connotes here the absence of enlightenment (Bodhi) of a person working towards that goal. In Mahayana Buddhism, on the other hand a bodhisattva is someone who seeks to put the welfare of others before their own, forfeiting their own enlightenment until all beings are saved. Such a person is said to have achieved a proto-enlightenment called bodhicitta.
It is not a synonym of Tirthankar, which refers specifically to certain arhats who have certain karmas that enable them to become teachers of Jainism. The Jain Navakar Mantra starts with "Namo Arhantanam".