A dome is a glass hood that older rigs used over the nail to keep vapour out of the room. A setup with one is called a domed nail, one without it domeless.
Why it disappeared: the dome had to come off, the nail had to be heated, the dome had to go back on — all of that around something red hot. Domeless nails and carb caps do the same job more safely and with better control of temperature.
If a dome turns up with an older setup, it is not a fault. Just count on finding a replacement for a broken one being a lottery these days.
→ What replaced it: Caps & Marbles