In a previous job I had it was the "Beer Truck" factor (you can see where our heads were) but the common term these days is Bus Factor. Put simply, it's your projects exposure to the risk of key members disappearing tomorrow. To be crude if your project can't survive a key member of your project being hit by a bus tomorrow then you have a very high "Bus Factor".

He points out that it's not just an open source problem, but something that companies should take into consideration as well. Technology groups are especially bad about having single developers consistently working on certain parts of a system. If the time comes that that developer can't do the work (bus, leaves company, etc), how much of an impact will that have? Cal suggests a few ways that it can be avoided including spreading the work around a bit more, moving developers to places in the codebase they don't know and avoiding the "black boxes" created by certain developers.