Vector is the third episode of the first season, and the third episode overall. It was released on February 4, 2005.
A deadly virus is spreading through LA and only Charlie's equations can determine how far and where the disease is headed. The CDC is also on the case, and Don has to contend with a potential turf war among the authorities.
In everyday usage, the term "angle" is often substituted, however, technically, a vector is "a quantity possessing both magnitude and direction". Such distinctions lead us into Charlie's territory.