Proportional means changing in step with something else. Proportional to the distance, a proportional response, proportional representation.
In voting it names systems where seats match votes across a country, which most democracies use and Britain largely does not. A proportional response in diplomacy is one matched to the provocation rather than exceeding it.
1adjectiveproportionalChanging in step with something else.
“Your pay will be proportional to the number of hours you work.”
proportional is a quality.