An enclosed, pressurised habitat located on the surface of a planet, moon or asteroid. To protect against cosmic radiation, smaller domes have thick shells and often are partially or wholly covered in regolith. Opaque domes of this kind require internal lighting, both for the comfort and convenience of the inhabitants and in order to support plant growth, especially crops.
Larger domes may be partially or wholly transparent, allowing light from the local star to enter for the purpose of plant growth. The atmosphere itself inside a large dome gives some protection against radiation; this is often augmented with magnetic or electrostatic shielding, or sometimes a layer of liquid water or ice.
On a slowly-rotating world, the day-night cycle may inhibit or prevent plant growth, so artificial lighting may be incorporated into the dome roof. Conversely, the transparent roof may become opaque if the insolation becomes too strong. A significant fraction of modosophonts are accustomed to a day-night cycle, and both domed habitats and rotating space habitats often incorporate some method for replicating this cycle. Many sophonts prefer to be active during the day, and find daylight psychologically comforting; other sophonts prefer the night, and sleep in the 'daytime', whether that daytime is artificial or not. On the other hand, many Terragen citizens are modified to do without sleep altogether, or ignore the local day/night cycles as a minor inconvenience.
Image from Ron Bennett
A large dome may entirely fill a crater, and several domes may be connected together to form a continuous habitat, with a flexible roof held up by air pressure.
Large, low, pressurised surface habitats are known as low houses; an even larger surface habitat, covering much or all of a planet (or other object) is known as a worldhouse. -----------------
The small, dry tidally-locked world of Fulno in the Resonance system is almost completely covered in individual dome habitats on the sunward side
Some small, dry, cool worlds hold numerous dome habitats, often with transparent roofs that can collect light and heat from the star and support a wide range of individual biomes. Domed habitats located on small worlds with minimal atmospheres can use the resources on the planet and maintain relatively easy communications between the individual domes; the natural gravitation of the planet means the habitat does not need to rotate in order to produce centrifugal gravity.
On the other hand, space-borne rotating habitats can gather material from a larger volume of space, and spacecraft approaching and departing such habitats require less delta-vee. A small planet can support several dozen large domed habitats, but in contrast the matter from such a planet, if disassembled, could be used to construct thousands of rotating habitats supporting a much larger population. For this reason, domed habitats are generally found in less populated systems far from the main centres of civilisation.