I've made this same comparison before, and people always love to argue about it, but it remains every bit as valid:
A US Supreme Court Justice once said: "I shall not today attempt further to define the kinds of material I understand to be embraced within that shorthand description ["hard-core pornography"], and perhaps I could never succeed in intelligibly doing so. But I know it when I see it, [...]".
While Potter Stewart did later say that this was not as solid an expression, nor one as memorable as he would have liked, the spirit of the statement holds quite true, especially when we speak of "griefing" and "ganking".
These are very much subjective terms, just as a definition of what constitutes legal obscenity is still very much a subjective matter.
While the argument that, for the sake of example a 5 on 1 fight might not seem "fair", let's look at a few possible outcomes:
1. The 5 dispatch the 1 with ease. The 1 was a willing participant. No harm, no foul.
2. The 5 dispatch the 1 with ease. The 1 was not a willing participant. This gets termed to be a "gank".
3. The 1 puts up a valiant fight, was a willing participant, no harm, no foul.
4. The 1 puts up a valiant fight, was not a willing participant, again, no harm, no foul. Could be called a "gank".
5. The 1 puts up a valiant fight, turns the tables on his attackers, and dispatches all of them. No harm, no foul, possible bragging rights.
6. The 1 is an incredible pilot, willing or otherwise, and decimates the 5. Again, no harm, no foul, and we never hear a word from any of the 5 around here, because who wants to own up to getting the floors mopped with them and all their friends by one lone pilot?
These just serve to illustrate the subjective nature of this particular term.
"Griefing" on the other hand... again, still subjective, and arguably a form of harassment, is perhaps a little easier to define within some manner of scope. Actions taken that are unwelcome, repeated, accompanied by badgering comms - for example, hunting a single player to a station and denying them departure by repeatedly attacking and destroying the same player, while telling them "We don't want Imperial/Federal scum in our system and you suck, learn to fly." - few would argue this isn't "Griefing", few would argue this isn't harassment. Fewer still might actually seek out this sort of encounter, if there were the one subjected to these kinds of attacks.
Again, a purely hypothetical situation, for the sake of making a point that is comprehensible, when discussing what is, by its nature, a subjective matter. We may not be able to define it clearly, but we do know it when we see it. What matters most is what we do about it and how we react to it.
