Sustainability is what people want to happen indefintely. No country has a GDP growth target less than about 2%, except when recovering from a recession. Thus the defacto defintion of economic sustainability is steady growth in total national GDP of a minimum of about 2% per year.
But this is the wrong defintion. Total national GDP doesn't tell you how much the average person's income is. Nor does it tell how many people are at the low end of the distribution of income and are thus starving. Nor is steady growth even possible forever. Steadily growing total GDP is thus a flawed goal that can lead a country, and the world, terriby astray.