hash rate

That means the total network hash rate, currently about 80,000 gigahashes per second, is used to determine that difficulty.

The benefit is that every "hash" that a miner submits contributes to the total hash rate of both currencies, and as a result, they both become more secure.

However, there are somewhat less overwrought writers who suggest there are only a couple of things which somebody with a 51% of the network hash rate computational power can do.

The end result: The entire network hash rate will likely depend almost entirely on the cost, initial hardware and cost of running, of whatever the most widely available mining hardware is.

Keep in mind a large portion of the current network hash rate is derived from GPU-based bitcoin miners, which have much worse megahash per second per dollars and megahash per second per - what's "J"?

The number six was chosen deliberately: it is based on a theory that there's low probability of wrongdoers being able to amass more than 10% of the entire network's hash rate for purposes of transaction falsification and an insignificant risk (lower than 0.1%) is acceptable.