Definition of 'Marginalism'
The study of marginal theories and relationships within economics. The key focus of marginalism is how much extra use is gained from incremental increases in the quantity of goods created, sold, etc. and how those measures relate to consumer choice and demand.
Marginalism covers such topics as marginal utility, marginal gain, marginal rates of substitution, and opportunity costs, within the context of consumers making rational choices in a market with known prices.
Investopedia explains 'Marginalism'
The idea of marginalism, and its value in establishing market prices as well as supply and demand patterns, was popularized by British economist Alfred Marshall in a publication dating back to 1890.
Marginalism is sometimes criticized as one of the "fuzzier" areas of economics, as much of what is proposed is hard to accurately measure, such as an individual consumers' marginal utility. Also, marginalism relies on the assumption of (near) perfect markets, which do not exist in the practical world. Still, the core ideas of marginalism are generally accepted by most economic schools of thought, and are still used by businesses and consumers to make choices and substitute goods.