quorum sensing

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A phenomenon in which a population of bacteria produces and responds to intercellular chemical signals whose concentration indicates the density of the population. When the density reaches a certain level, genes are expressed that allow for coordinated group behavior, as in bioluminescence or the formation of biofilms.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A proposed method of communication between bacterial cells by the release and sensing of small diffusible signal molecules.