The Bus Factor is the number of people that could be run over by a bus before a software project would have code nobody is familiar with (or for the project to be neutralized).
Linux has a bus factor of 1.
A measure of employee value and commitment relating to the amount / cost of work that would suffer as a result of person X being run over by a bus.
"Crikey was that Willy's idea as well? I dunno what we'd do without him"
"Yeah, he has a really high bus factor"