When researchers study a specific well-defined population over time, sampling different people at each data collection point, it refers to a

Studying a specific, well-defined population over time with different people each round is a cohort study.

Each wave samples fresh members drawn from the same defined cohort.

A cross-sectional study gathers data at a single point in time only.

A panel study follows the very same individuals across the waves.

A retrospective study looks backward at past events and records.

Cohort designs trace change while keeping the defining population fixed.