What is the name of the relationship between two quantities that vary together over a period of time?

Answer: Correlation

Correlation is the relationship between two quantities (variables) that vary together over a period of time? The relationship may be positive (where the two quantities will have same high value or low value). A negative correlation is when one value is high while the other one is low and vice versa. Correlation may range from + 1to - 1 only.