Definition

Confounding variable

A third variable that influences both the presumed cause and the outcome, creating a spurious or distorted association.

Ice cream sales correlate with drownings because summer drives both — temperature is the confounder. In research: education confounds the link between coffee and health; disease severity confounds treatment comparisons in observational data.

Randomization is the definitive cure, balancing unknown confounders as well as known ones. Without it, options are measurement plus statistical adjustment (regression, matching, stratification) — which only handle the confounders you thought of and measured well.