Where does 'efficacy' come from?
It represents how much light can be generated from an amount of electricity. Read more about efficacy on wikipedia. The math looks like (lumens * runtime) / (battery watt hours). Better circuitry and better LEDs will improve the efficacy. The number reported is the highest value for any mode. Good manufactures will say what type of battery they used. If they don't, then I've had to guess and the efficacy may be a little off.
This only works for regulated flashlights. If you see a wildly absurd number (above 200 lum/watt) it probably means the flashlight is unregulated.What is 'beam angle'?
It is crude estimate of throw-y vs flood-y a flashlight is. It starts with the candela and the lumens. Assuming the candela value is consistent across the entire beam, you can then find the beam angle. Lots of lumens but a low candelas means a very wide angle floody beam. Lots of lumens with a high candelas means a very tight throwy beam. Details about the math are on wikipedia.
The number will not be entirely accurate for beams with lots of spill.What is all this about "IPX8"? Everyone says "IPX-8".
Read about the Ingress Protection Rating. The name of the rating is 'IP'. The X specifies that it was not tested for dust resistance and is not part of the name.What does (turbo) mean in the description of the modes?
Some manufactures will overdrive the light for a few minutes before stepping down to proper regulation. This makes the light look much more impressive by ANSI standards, but could be considered misleading.