Dashavatara refers to the ten principal Avatars. In Vaishnava philosophy, an Avatar most commonly refers to the 'descent' and dasa refers to 'ten' in number. The ten most famous incarnations of Vishnu or sometimes Krishna are collectively known as the Dashavatara. This list is included in the Garuda Purana (1.86.10-11) and denotes those avatars most prominent in terms of their influence on human society.
Buddha, the ascetic prince is listed as an avatar of Vishnu in many Hindu scriptures including Bhagavata Purana, Bhavishya Purana, Narasimha Purana etc. With the departure of Krishna, Kali Yuga sets in, in this age, the true devotion to Vedas was replaced by empty rituals. To enlighten the world in such times, Vishnu descended the earth as Buddha, the enlightened one.