The term which Major uses to address the other animals is 'comrades'. In a general sense, 'comrade' means friend, partner, someone on the same footing as oneself and part of the same group. Major uses it as he wishes to include all the other farm animals in his great vision of a future society in which all animals of the nation shall be free. He encourages all the animals on Mr Jones's farm to share in that ideal. Therefore he addresses them as 'comrades'.
Major's use of the term 'comrades' also has a deeper significance when we consider Orwell's reasons for writing Animal Farm. His portrayal of a idealistic society that goes badly wrong is an allegory of dictatorship. In particular, and quite deliberately, it recalls Stalinist Russia in the 1930s and 40s, when Josef Stalin was head of the ruling Communist Party. The Communists had initially been fired by elevated notions of a socialist nation where everybody would be truly free and equal, and whose members addressed each other as 'comrade', as a reminder of their solidarity and dedication to making this vision a reality. However, this soon degenerated into interior power struggles and the eventual emergence of Stalin who gained supreme control and meted out savage oppression to both fellow-Party members as well as to the country at large, just as Napoleon in Animal Farm ends up ruthlessly oppressing his fellow-animals. Yet, to the end, when the rest of the animals have become practically enslaved by the pigs and their supporters, the dogs, the pigs still insist on calling them 'comrades', in a mockery of the original ideals on which Animal Farm was founded.