Final answer:
When we move along a demand curve, price is the factor that is not held constant. Variation in price is what causes the movement along the demand curve, as opposed to shifts in the curve which are caused by changes in income, tastes and preferences, and other factors.
Step-by-step explanation:
In moving along a demand curve, the factor that is not held constant is the price of the good or service. When we move along the demand curve, we are looking at how the quantity demanded changes in response to a change in price, holding all other factors such as income, tastes and preferences, and quantity demanded, constant. It's important to know that a movement along the demand curve differs from a shift in the demand curve. A shift would occur if there were changes in non-price factors such as income, tastes and preferences, the composition or size of the population, the prices of related goods, or expectations. These changes result in a new demand curve, which is either to the right (an increase in demand) or to the left (a decrease in demand) of the original demand curve. Therefore, when analyzing the movement along a demand curve, it's the price that varies, not the constant factors that would cause the demand curve to shift.