Confounding variable
Etymology
“Confound” comes from Latin “confundere,” “to pour together, to mix up” — the confounder mixes up cause and effect. The concept is central to experimental design, where randomization and control groups exist largely to neutralize confounding variables.
noun phrase · /kun-FOWN-ding VAIR-ee-uh-bul/
A hidden third factor that influences both the supposed cause and the effect, creating a misleading link between them.
What it names
The lurking variable that makes two things look causally connected when both are really driven by something else.
Example
Ice cream sales and drownings rise together, but the confounding variable is summer heat, which drives both.
Related idea
A confounding variable secretly causes the correlation you observe; it is the concrete mechanism behind the warning that correlation is not causation, and controlling for it is what separates a real effect from a spurious one.