In Buddhism, a buddha (Sanskrit: Awakened) is any being who has become fully awakened (enlightened), and has experienced Nirvana.
In the Pali Canon and the Theravada tradition, the term 'buddha' usually refers to anyone who has become enlightened (i.e., awakened to the truth, or Dharma) on their own, without a teacher to point out the Dharma, in a time when the teachings on the Four Noble Truths or the Eightfold Path do not exist in the world. By comparison, those who awaken due to the teachings given by a Buddha are known as Arahants, a title also applied to Buddhas. Arahants and Buddhas are the same in the most fundamental aspects of Liberation (Nirvana), but differ in their paramis.
In the Mahayana tradition, the definition of Buddha extends to any being who becomes fully awakened. The Theravada Arhant would be considered a kind of Buddha (although not generally by Mahayana Buddhism itself) in this Mahayana sense, and this usage also occurs once in later (12th century) Theravada literature.
Buddhists do not consider Siddhartha Gautama to have been the only Buddha. The Pali Canon refers to many previous ones (see List of the 28 Buddhas), while the Mahayana tradition additionally has many Buddhas of celestial, rather than historical, origin (see Amitabha or Vairocana as examples). A common Buddhist belief across all Buddhism is that the next Buddha will be one named Maitreya (Pali: Metteyya).