Correlation
Etymology
A relatively modern compound, from Latin "com-" (together) plus "relatio" (a bringing back, relation); the precise statistical sense was developed in the 19th century, largely by Francis Galton and Karl Pearson, who gave it a number. The word simply names things related together, carrying no claim about what drives what.
noun · /kor-uh-LAY-shun/
A statistical relationship in which two things tend to vary together.
What it names
A handle for the pattern that two measures move in step — the raw observation before any story about why.
Example
There’s a strong correlation between shoe size and reading ability in children — because both rise with age.
Related idea
The whole point of the word lives in its contrast with causation: correlation says two things move together; causation says one makes the other move. A hidden third factor, or pure chance, can produce correlation with no causal link at all — the shoe-size example is exactly this.