the.com/control group

means The baseline set of subjects in an experiment that doesn't receive the treatment being tested, used as a point of comparison to see what changes are actually caused by it.

from From 'control' in its scientific sense of 'a standard of comparison used to verify an experiment's results,' a usage that grew out of the older, more general meaning of control as 'checking or regulating something against a fixed standard' — the same root sense behind a 'control' on a ledger or a 'controller' who checks accounts. That older sense traces back through French contrerôle, literally a 'counter-roll' or duplicate register kept to check the original. The experimental 'control group' itself is a relatively modern formalization, tied to the rise of statistics and experimental method, where scientists needed language for the untreated group held up against the treated one as the fixed point everything else is measured from.