Griefing is a new term for me, so I looked up the meaning of griefing using Google. And from the gist of the myriad of definitions out there, I gathered this.
Is my opinion and nothing more, this is what I understand from what I read.
"It's pretty much a player of and on-line game that intentially goes out of there way to do something that results in reducing or ultimately destroying the gaming experience for another player. The ultimate goal is to cause the player on the receiving end grief, whether intentional or not. So, I guess we should really call the Pirates Griefers."
So, I guess the various examples that have been given would be considered griefing whether the server allows or or disallows the behaviours like attacking a players base while they're not at home, steeling their stuff or plain harassing them in other ways including on forums or other.
Here are URLs to my search results for you to look up.
Google
Urban Dictionary: griefing
Urban Dictionary: griefer
Griefer - Wikipedia, the free encyclopedia
Tutorials/Griefing – Official Minecraft Wiki
And there are so many more.