Bioaccumulation is the gradual accumulation of a certain chemical into the living tissue of an organism from its environment. This accumulation may result from direct absorption from the environment or from ingestion of food particles.  Glossary]
The concentration of bioacumulation substances usually increases in with the age of an organism. Bioaccumulation is one of the factors used to asses the environmental hazard of a chemical. Chemicals with a higher tendency towards bioaccumulation form a greater hazard.