Longitudinal Data:

Longitudinal data refer to observations of a given unit made over time. Such data can usually be represented as a time-series and, therefore, they are often referred to as time series data.

A simple example of longitudinal data is the average gross annual income of, say, 1000 households randomly chosen from New York City for years 1991-2000. Such data could be represented as a time series of 10 values (each value being the average gross income of all 1000 households for one year).