Butter (alternatively spelled Budder) is what SkyDoesMinecraft nicknamed gold and uses the nickname when finding a gold ore or get a gold weapons.
Originally, Seananners created the nickname "butter", but it grew in popularity once SkyDoesMinecraft started using it. SeaNanners fans originally said sky copied SeaNanners, but in the end the issue died down.
Sky never calls gold by its original name. He always refers to it as "butter" and everytime somebody says "gold" while they are playing with Sky, he will probably respond by saying something like, "What's this 'g-word' everybodys saying?". Butter has become a symbol for Sky and is the "opposite" of squids, since of recent Minecraft Mod Showcase videos.
Sky, on an episode of "Bridges", told his fans to stop correcting other YouTubers when they call butter "Gold." He says it is rude and that he still loves butter but he wishes for people to stop. JeromeASF with a laugh reiterated on it saying that, "Yes. You merely dislike their video."
Sky also uses butter as the term for a gold sword and all gold tools, and upon finding a weapon/tool in a pvp minigame, he will most likely say something along the lines of I got a butter sword! or other alternative versions.
Butter also appears in various fanmade wallpapers for Sky, and fans also use it if they can or want to, due to the fact that a lot of fans want to show how much they appreciate his term or just like the nickname.
Butter also is used as a weapon in Sky Hunger Games, called a butter slapper, which, on right click, will throw snowballs at medium rate speed, and are a very popular weapon used when got hold of, and they don't run out of the snowballs chucked. Blue Monkey, the animator of TC was once killed in-game on Sky Hunger Games by a player with a butter slapper.
Although fans of SkyDoesMinecraft often used the term, it has become negatively associated with trolling and obnoxious fanboys of the term. It still is used positively a lot, though.