The term biomagnification refers to the process by which the concentration of a non-degradable pollutant increases in organisms as it moves higher up in the food chain. This means that as smaller organisms consume contaminated food, they absorb and store the pollutant in their tissues. When larger organisms consume these smaller organisms, they ingest a larger amount of the pollutant. This process continues up the food chain, with each level ingesting and accumulating more of the pollutant. As a result, organisms at the top of the food chain, such as apex predators, can have very high concentrations of the pollutant in their bodies.

The other options provided do not accurately describe biomagnification. Option 1, increase in body weight, is incorrect because biomagnification does not directly relate to an increase in an organism`s weight. Option 2, uncontrolled growth of harmful organisms, is also incorrect as biomagnification specifically refers to the accumulation of pollutants, not the growth of organisms. Option 4, increase in the number of bacteria in a culture medium, is unrelated to biomagnification. Therefore, option 3, accumulation of increasing amounts of a non-degradable pollutant through the food chain, is the correct definition of biomagnification.