Is it truck factor or bus factor?
agile
Solution
Wikipedia says bus number is "more commonly known as truck number" But in the US, "hit by a bus" is practically an idiom, while "hit by a truck" is not (although either phrase is easily understood.) Regarding high/low being good, the wikipedia article says:
"High bus numbers are good (with the best case being equal to the number of developers on a project). This means all developers understand the codebase more or less equally. A low bus number (especially, one) represents a high risk."
I'd add to what @cartoonfox said: Promiscuous pair progamming is a good way to distribute critical knowledge around a team so that the truck number is as high as possible. If you don't swap pairs often and with many different team members, knowledge isn't distributed very quickly.
Problem
It seems that both these terms get thrown around a lot. Both, I think, describe the same idea. Which was established first? Also, it seems like some people describe it as a good thing to have a low x factor, while others describe it as a good thing to have a high x factor. Which is it?