The term "multivesicular" refers to something that is composed of multiple vesicles or small sacs. This can describe a structure, such as cells within an organism, which have several membrane-bound compartments, or it may refer to a substance with many tiny bubbles or air pockets. In biology, the term "multivesicular" is often used in relation to endocytosis, where cells take in materials by forming vesicles around them, and these vesicles then fuse together to form larger compartments within the cell.