Demand is the quantity of goods consumers are willing and able to purchase at various prices in a given period. Distinction: Desire (want without ability), Demand (want + ability to buy). Key characteristics: Negative relationship with price (quantity demanded falls as price rises), measured at specific price points, has a time dimension. Law of Demand: As price increases, quantity demanded decreases, ceteris paribus (other things constant). Reasons: Substitution effect (switch to cheaper alternatives), Income effect (higher price reduces purchasing power). Exceptions (Giffen goods): Essential items where rising price increases demand due to weak income effect. Example: Wheat in India during crop failure. Demand curve: Downward sloping, typically linear or curved. Movements vs. shifts: Movement = change in quantity due to price change; Shift = change in entire demand due to non-price factors. ICAI tests: Law application, exceptions, movement vs. shift distinction. Exam tip: "Demand curve slopes downward" is fundamental; any question suggesting otherwise contradicts law unless mentioning Giffen goods.