What consists of a zombie movie?

I’m curious in your opinion, or any actual fact which may exist about this.
We all think one thing when thinking of zombie flicks…but while searching for a list of zombie movies for a user here, I remembered something I addressed long ago…what exactly would consist of a zombie movie?
I see a lot of ’‘zombie’’ movies that don’t always deal with zombies, or, that propose beings and monsters that are reminiscent of zombies, but technically are not.
Zombies are simple, but they can be very complex. A Night of the Living Dead zombie is not the same as a 28 Days Later zombie. The Crazies kind of borrows the zombie concept, but these are not zombies. Pet Semetary does include the dead coming back to life, but the undead have many shapes. Why would a movie like that be considered a zombie movie by some?

I think we need to include a subgenre for horror flicks which borrow close zombie material but still are not zombie movies. Invasion/Infection movies perhaps.

I think it all depends on what a zombie is to people, despite that the zombie is very well anchored and defined in the entertainment medium… I’ve got my own idea; a zombie needs to be a returning corpse, (Or living human infected by another zombie.) and want only one thing; human flesh. Doesn’t matter how they do this, although I much prefer the shambling, slow zombies…but if those two criteria are met, then it’s good.
It gets complicated sometimes though…Pet Semetary undead to me are not zombies, because they can talk, trick people and all.
Zombies with guns and speech also make me question their authenticity…see where I’m getting at? Not to mention the concept idea for certain films I mentioned earlier.

What do you think constitutes a zombie movie? Are the monsters more important than the concept of the film, or vice versa? As we’ve seen, zombies can be a lot of different things, and can do a lot of different things…is there fact beyond generalism, or is it all opinion? What do you think?