Correlation Coefficient
Etymology
“Correlation” is from Latin com- (together) plus relatio (a bringing back). The specific measure, Pearson’s r, was developed by Karl Pearson from Francis Galton’s ideas around 1895; “coefficient” combines Latin co- (together) with efficere (to bring about).
noun phrase · /kor-uh-LAY-shuhn koh-uh-FISH-uhnt/
A number from -1 to +1 measuring how tightly two things move together.
What it names
It puts a precise strength on a relationship, distinguishing a firm link from a barely-there one.
Example
A correlation coefficient of 0.9 between study hours and grades is strong; one of 0.1 means the two barely track each other at all.
Related idea
A high coefficient still shows only correlation, not causation, and it captures only straight-line relationships — a perfect curved link, like a U-shape, can score a coefficient near zero.