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. 
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.