A steaty state economy is an economy of stable or mildly fluctuating size1.
Often discussed in relation to ecology and economic growth.
Economic growth must be separated from economic development. The former implies monotonic growth of consumption and production. The latter implies redistribution, changes in uses, improvement of methods of extraction, increasing efficiency and so forth.
A steady state economy implies constant populations of people and material/energetic throughput.
This throughput might change due to the aforementioned development and related increase in efficiency, but due to physics and the fundamental, non-negoitable laws of thermodynamics there is a maximunm size given a certain ecosystem society is to be in.
Material throughput and futurism
Tegmark is however more futuristic and thinks on longer time scales than do Casse. Tegmark argues that the physical limits of possible development are still very far off, and current human achievements don’t really go that far. Casse seems to be more of the Small is beautiful type.
Predominant macroeconomic policy goals
Neoclassical economics → Economic growth
Ecological economics → Steady state