1. In cloud physics, an obsolete term denoting any process that converts the numerous small cloud drops into a smaller number of larger precipitation particles. When so used, the term is employed in analogy to the coagulation of any colloidal system. The process can take place at temperatures both above and below 0°C for supercooled drops. See coalescence. 2. Similar to accretion. 3. The process whereby aerosol or colloidal particles collide with each other by Brownian motion and coalesce (liquid) or aggregate (solid).