The "bus factor" is the minimum number of team members that have to suddenly disappear from a project before the project stalls due to lack of knowledgeable or competent personnel.

The expression "hit by a bus" describes a person either dying or more generally disappearing suddenly from the project. It is used to describe hypothetical future disappearances in a darkly humorous way. Team members do not literally have to get "hit by a bus" for the "bus factor" to apply—any number of events could occur in which a team member could be suddenly and substantially prevented from working on the project. This could include a person taking a new job, going on parental leave, or changing lifestyle or life status.

For instance, say a team of 30 people produces bread in three necessary steps: mixing ingredients, kneading the dough, and baking. Ten people know how to mix ingredients, all 30 people know how to knead the dough, and 5 people know how to bake. If all 5 people who know how to bake disappear, then the team cannot produce bread, so the team's bus factor is 5.

There is a rare alternative definition for the bus factor, namely: the number of people who are indispensable for the project.[2] In other words, it is the minimum number of people who are a single point of failure. If using this definition, then a high bus factor is considered a bad thing (since the loss of any person included destroys the project), and zero is considered the ideal bus factor.

An early instance of this sort of query was when Michael McLay publicly asked, in 1994, what would happen to the Python language if Guido van Rossum were hit by a bus.[9]

A recent study calculated the bus/truck factor of 133 popular GitHub projects. The results show that most of the systems have a small bus factor (65% have bus factor ≤ 2) and the value is greater than 10 for less than 10% of the systems.[10][11]