NSES Content Standard Unifying Concepts and Processes:  
Constancy, change, and measurement 
Grades K-12, page 118 

Rate involves comparing one measured quantity to another measured quantity, for example, 60 meters per second. Rate is also a measure of change for a part relative to the whole, for example, change in birth rate as part of population growth. 

Benchmark 9B The Mathematical World: Symbolic Relationships
Grades 6-8, page 219
Mathematical statements can be used to describe how one quantity changes when another changes. Rates of change can be computed from differences in magnitudes and vice versa.

Benchmark 9B The Mathematical World: Symbolic Relationships
Grades 9-12, page 220
In some cases, the more of something there is, the more rapidly it may change (as the number of births is proportional to the size of the population). In other cases, the rate of change of something depends on how much there is of something else (as the rate of change of speed is proportional to the amount of force acting).

Benchmark 11C Common Themes: Constancy and Change
Grades 6-8, page 274
Symbolic equations can be used to summarize how the quantity of something changes over time or in response to other changes.