I promised I would get to this eventually. Here you go. More boring junk. This post will be updated as new terms are brought into play, and will be linked to from all new tutorials going forward.
ACE - Area Control Error
A numeric value, in megawatts, that a power company's Control Area is under or over-generating at any given moment.
AGC - Automatic Generator Control
A computer program that determines the best reliable and economic means to move all of the power plants, within a particular power company's area, up and down in order to keep energy production costs as low as possible, reduce wear and tear on the power plants, yet maintain adequate system stability. The AGC program is used to automatically control power plants in real time, but can be overridden easily in an emergency.
MSSC - Most Severe Single Contingency
The single biggest, baddest, most horrible thing than can happen to a power system within a defined area, at least within the constraints of the imaginations of that system's dispatchers and planning engineers. If you have four adjacent power systems tied together to form one grid, each of those four will have their own MSSC, but the worst of all four is also that grid's collective MSSC.
MW - Megawatt
A unit of energy, equal to 1,000 kilowatts. There is a slight differentiation between an instantaneous MW flow rate and the amount of megawatt hours (MWh) delivered over a period of time. For example, if 100MW flows across a line for only 30 minutes, and then the line trips out of service, that line still successfully delivered 50MWh during those first 30 minutes (half of the 100MWh it would have delivered if it had stayed in service). For reference, a typical single family residential home may consume something in the neighborhood of one megawatt of energy over the course of a month.
NAI - Net Actual Interchange
A numeric value, in megawatts, that a power company's Control Area is receiving or delivering collectively from all of its neighbors at any given moment.
NSI - Net Scheduled Interchange
A numeric value, in megawatts, that a power company's Control Area is supposed to be receiving or delivering collectively from all of its neighbors at any given moment.
SF6 - Sulfur hexafluoride gas
An inert, relatively harmless gas used inside of high voltage circuit-interrupting equipment such as circuit breakers and circuit switchers to extinguish the electrical arc created when breaking contact between energized components. Very high voltage switching devices, if opened under load without SF6 capability, are likely to produce dangerous and potentially destructive electrical arcs which can destabilize the system, cause other equipment to trip offline, burn things up.... all right, enough, you get the idea.
Did I miss anything from the posts I have already dumped into your lap? Under or over-explain anything? Comments as always welcomed...