Can you explain what economists mean by the Law of Demand?

Answer:
The Law of Demand is summarized by Professor Karl Case in the following definition:
"There is a negative relationship … shown by the downward sloping demand curve … between price and quantity demanded. As price rises, quantity demanded falls; and as price falls, quantity demanded increases …"
(Case, K; Fair, Strain, Veall, Principles of Microeconomics, Scarborough: Prentice-Hall, 1998, p. 75-8.)

Try to understand and recall all aspects of this definition because distinguishing between a negative and a positive relationship is essential to your work in economics. Note that demand reflects trade offs between goods. When the price of a good gets very high, for example, we may consider an alternative, cheaper good as a reasonable trade off. It also assumes a choice in favour of a particular good; the consumer has already made up his or her mind on the utility of the good, and is prepared to pay the price.

The British economist, Alfred Marshall, first coined "Demand and Supply" in his 1890 textbook called Principles of Economics. He made a pair of scissors with two blades his analogy for "Demand and Supply," interacting at a central point, which he called the equilibrium or market-clearing price. Marshall also noted that his use of the word "law" was not physical science. Rather, it is social science, and denotes a reasonable expectation: "The term 'law' means nothing more than a general proposition of tendencies … more or less certain, more or less definite …." (Quoted in Karl Case, Principles of Microeconomics, op.cit., p.77.)