Longitudinal data refers to data collected over a period of time from the same individuals or subjects. This type of data allows researchers to observe changes and trends within the same group of individuals over time, providing valuable insights into development, patterns, and relationships that may not be captured through cross-sectional data. Longitudinal studies are particularly useful for studying issues such as growth and development, changes in health outcomes, and the effects of interventions or treatments over time. By tracking individuals over a period of time, researchers can better understand how various factors influence outcomes and behavior.