In the frame of current article it is researched the definition of sustainability facilitating sustainable development of public transport system. This research will give a new look to definition of sustainability, will be the source of new scientific ideas and ways of thinking about the public transport system and its sustainable development. Analysis of definitions and interpretations of sustainable development and sustainability and summarizing of principles of sustainability and sustainable transport system, as well as definition of indicators of environmental sustainability in transport system and assessment of environmental impact indicators for transport in smart city era, which can help to formulate improvements of sustainability definition facilitating sustainable development of transport system, will give a huge positive impact to development of transport area using sustainability principles. Solutions for sustainability are also offered in the article. It is the first article in the new cycle with research and discussions about sustainability and improvement of its definition, about transport system sustainability and related topics.