"Longitudinal associations" refers to the relationship between two variables or factors over a period of time. It means observing how changes in one variable are related to changes in another variable as time passes by. Full definition
This study examined longitudinal associations of prenatal exposures as well as childhood familial experiences with obesity status from ages 10 to 18. (link.springer.com)
Diabetes problem solving by youths with type 1 diabetes and their caregivers: measurement, validation, and longitudinal associations with glycemic control. (nejm.org)
We examined longitudinal associations and concurrent between - person and within - person associations. (link.springer.com)