When something reaches the threshold of it's limits. This can apply to things, people, events, or concepts. (Urbandictionary.com)

1. the minimum amount (of something) required to start or maintain a venture; "the battle for the computer market has now reached critical mass"; "there is now a critical mass of successful women to take the lead"; "they sold the business because it lacked critical mass"
2. the minimum mass of fissionable material that can sustain a chain reaction
Source(s):
http://dictionary.reference.com/browse/critical%20mass
http://www.urbandictionary.com/define.php?term=critical+mass

Permalink | Report

--quote--

Critical Mass is a monthly bicycle ride to celebrate cycling and to assert cyclists' right to the road. The idea started in San Francisco in September 1992 and quickly spread to cities all over the world. This site attempts to be the most complete guide to all Critical Mass rides around the globe.

--endquote--

It's also a big pain in the rear end for most people. Riders in CM have been known to cause some serious problems. (My opinion.)
Source(s):
http://critical-mass.info/

Permalink | Report

In nuclear physics it is the minimum mass of fissionable material which needs to been brought together to produce a self-substantiating nuclear reaction.

If the mass is just over this limit you have a nuclear power plant. If the mass is substantially greater than the critical mass then you have a nuclear weapon.
Permalink | Report