> The name Gigafactory comes from the word “Giga,” the unit of measurement representing “billions.” The factory’s planned annual battery production capacity is 35 gigawatt-hours (GWh), with one GWh being the equivalent of generating (or consuming) 1 billion watts for one hour. This is nearly as much as the entire world’s current battery production combined.

via https://www.tesla.com/gigafactory

Doesn't explain why they call Buffalo's operation a "Gigafactory," but that's the origin of the name for the more well-known Nevada-based Gigafactory.