1. Economics has been defined in various ways by different economists over time. Early definitions by Adam Smith and others focused on wealth, while Marshall defined economics as studying how people cooperate to meet material needs and maximize welfare. 2. Robbins provided an influential definition, defining economics as studying human behavior with limited resources that have alternative uses. This focused on scarcity rather than welfare. 3. Modern definitions, like those from Keynes and Benham, emphasized factors that determine a country's national income, output, employment and economic growth stability over time.