When it comes to mathematical and scientific contexts, the term "directly proportional" is commonly used to describe a relationship between two variables in which an increase or decrease in one variable leads to a corresponding increase or decrease in the other variable. However, in order to add variety and depth to your writing, there are several synonyms that can be used instead of "directly proportional". Some examples include "in direct correlation", "commensurate", "concordant", "linear relationship", "parallel" or "tied together". By incorporating these alternatives, you can enhance your language and effectively convey the concept of a direct relationship between two variables.