What is Aggregate Demand?

Aggregate demand or AD refer to the total value of final goods and services people are planning to buy with given income level during given accounting period. It is the same as Aggregate Expenditure which includes all the expenditure that different sections of an economy are willing to incur.
Just like aggregate expenditure AD is also planned i.e. it is the total value of expenditure various sectors (households, firms, rest of the world and the government) are willing to incur in a given accounting period.