1. a large, gluelike molecule that cannot pass through the plasma membrane of a cell. When injected into the peritoneum, colloids attract and retain water by osmosis.

2. a system consisting of two or more phases in which one phase (the dispersed phase) is distributed throughout the other phase (the continuous phase). A sol, for example, consists of large molecules or small solid particles distributed in a liquid, as when starch is mixed with water. —colloidal adj.