For many years, the survivors of The Walking Dead have traveled through a world where the majority of the population is undead. And not once during that time has the antagonistic threat ever been referred to as "zombies," with the characters mostly using the term "walkers" to describe them. Creator Robert Kirkman explains below that it's because pop culture zombies aren't a thing in this world, and he later shares how little the creative team follows the "no zombies" rule behind the scenes. In Kirkman's words:
We wanted to kind of give you a sense that The Walking Dead takes place in a universe where zombie fiction doesn't exist. No one inside The Walking Dead has seen a Romero movie, so they can't get the rules from that. We felt like not having people use that word would separate it from that a little bit, make it a little more clear.
Robert Kirkman conceived The Walking Dead as both a zombie action drama and a commentary on the undead subgenre, in that his story wouldn't just end as all zombie movies do. Kirkman's characters would have to continue living in this world, and without any of them having endured a lifetime of entertainment choices that featured zombies rising from the grave with a hunger for brains. Everybody's got a friend that likes to yell at the TV screen (or theater screen) to complain about characters in horror projects not doing common sense things, but in the case of Rick & Co., zombie sense isn't common at all.
It would actually be pretty crazy if someone did use the word zombie at some point, since none of the other characters would have any idea what they were talking about. Like when Denise got shot through the back of the head with that crossbow bolt, and her words started to get weird and slurry, or when Glenn was trying to talk to Maggie after the top of his head got caved in. If you hear the word "zombie" on The Walking Dead, it's possibly a sign of someone suffering from brain damage or a stroke.
But don't be fooled into thinking that Kirkman, as well as showrunner Scott Gimple, has a gold-plated plaque on the wall that says "Zombies Shall Never Be Named. Punishable By Death." During his interview with Conan O'Brien on TBS' Conan, Kirkman explained how that distinction is solely for The Walking Dead characters.
I still call it a zombie show myself. It's kinda funny, in the scripts - you know, it's just the dialogue that we keep it out of - and so it's 'The zombies come here,' and 'The zombies come there.' Then, all of a sudden, I'll slip a 'zombie' in the dialogue and go 'Ooh, God! I gotta correct that." But it's real haphazard behind the scenes.
It would be kind of amazing if, by the end of The Walking Dead, one of the last living characters is the one who coins the term "zombie" to describe the walkers. There would probably be a conflict as far as the etymology goes, but I don't think word origins are the biggest source of fan revulsion on this show. It could happen.