A Zone is the name used in early games of the Sonic the Hedgehog series to refer to the distinct playable levels. Each Zone is broken down into a number of Acts, each with its own finishing point. Most Zones have either two or three Acts, although some have one or four.
The term was used in this context in Sonic the Hedgehog (1991), Sonic the Hedgehog 2, Sonic the Hedgehog 3, Sonic & Knuckles, Sonic 3D, and others. (Sonic CD changed the terminology somewhat, using "Rounds" in place of Zones and using the word "Zone" to refer to what was usually called an Act.)
Each Zone usually has its own unique landscape, palette, and sprites, with its own set of enemies to defeat and obstacles to overcome. Each zone divided into 2 or 3 "Acts" and at the end of the first few Acts is a Bonus Plate which marks the end of the act as well as grant a bonus depending what the post sign displays. The final act is usually shorter and ends with a boss fight (usually against Doctor Eggman in a machine of sorts). However, some Zones will have different appearances across their separate acts (for example, Act 3 of the Scrap Brain Zone in Sonic 1 resembles the earlier Labyrinth Zone more than Scrap Brain's first two Acts).
It is not clear within the games whether the name of the Zone also relates to the location where the level is set (in some instances, such as the Doomsday Zone, which takes place in space, it is quite clear that this is not the case), but in many cases the name of the location is taken to be the same.
Zones are dimensionally phenomenal locations only present on an alternate Earth. Known to have there own unique environments that works with or just next to neighbouring zones despite complications or contradictions the zones would have on one another. There are only a few capable navigating through these zones such as Sonic, his friends or vehicles made for the situation.
Zones may also not follows the laws of physics but usually go by nature being that they can vary to geographically impossible sizes, but still based on the geology. Almost all the zone can still have identifying features which makes them recognised as zones such as unique land or rock formations, such as loop-de-loops, cork-screws and even parts of it's earth that levitates against gravity without the need of supports.
Angel Island (not the zone) is an example of different variety of zones, each of unique environments. Sandopolis Zone has a whole desert region which is like Ancient Egypt with the Echidnan ruins while IceCap Zone is a frozen mountain surrounded by glaciers. Not only do these environments contradict themselves but it's all located on an island that levitates as high as around strato-mesosphere.
Zones are prevalent in BearfootTruck's universe as well, with most of them situated in the Kingdom of Mobius. Being one of the most biodiverse countries on the planet, there are several zones in this country, each with their own environment. The southern & western quadrant is largely verdant grasslands & forests, including Green Grove Zone, Green Hill Zone & Emerald Hill Zone (both located on South Island). Interestingly enough, the Great Forest is not considered a zone, but is referred to as such by some of the locals. Seaside Hill Zone is located in the southeast. Along the border of the Republic of Shamar, desert environments such as Oil Desert Zone are dominant, except for Azure Lake Zone & Rusty Ruin Zone. The northern part of the country is an interesting case, as it includes Volcano Valley Zone in the northwest, Rail Canyon Zone in the north & Diamond Dust Zone in the northeast. The latter is the subject of a border dispute with the Roseinian Empire.
Other zones are located on Angel Island, including Hydrocity Zone, Marble Garden Zone & Lava Reef Zone. The Federal Republic of Columbia even includes a few zones of its own, such as Emerald Coast Zone on the west coast.