Demand
Plain definition
Demand is the full relationship between a good's price and the quantity buyers are willing and able to purchase, holding everything else constant (ceteris paribus). It is usually drawn as a downward-sloping curve: lower price, higher quantity demanded.
In economics
Demand sits at the center of price theory. A change in the good's own price causes a movement along the curve (a change in quantity demanded), while changes in income, tastes, prices of related goods, expectations, or the number of buyers shift the entire curve. Demand explains why markets respond to prices the way they do.
Example
When coffee shops raise a latte from 150 to 200 rupees and daily sales fall from 400 to 300 cups, that is a movement along the demand curve. But if a health study praises coffee and the same price now sells 500 cups, demand itself has shifted right.
The classic mix-up is demand versus quantity demanded. A price change causes a change in quantity demanded — a movement along the curve. A non-price change (income, tastes, substitutes) causes a change in demand — the whole curve shifts. Saying demand fell when you mean fewer units were bought because price rose mangles the model's language.