Hmm i can tell u : a word or phrase that means exactly or nearly the same as another word or phrase in the same language ,for example: close is a synonym of shut . "the west was a SYNONYM for the ussr".

population growth: refers to the change in population over a unit time period.

population density:number of people living in a certain area at an average is callen population density.

ahhh by reading this above information you can come to a conclusion weather both are SYNONYMS or not.
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Population growth means increase in population .where as population change can either be increase or decrease in the population.