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.
Kurma, the tortoise, appeared in the Satya Yuga. The tortoise Incarnation is the second incarnation of Vishnu. When the devas and asuras were churning the ocean in order to get the nectar of immortality, the mount Mandara they were using as the churning staff started to sink and Lord Vishnu took the form of a tortoise to bear the weight of the mountain.