flatlander (plural flatlanders)
- (pejorative) A person who lives at low altitude (used by those living at higher altitudes)
- In and to the west of the Rocky Mountains, it refers to anyone from the East.
- In the Appalachian region, it refers to any outsider.
- In northern central Pennsylvania, it refers to people from southern Pennsylvania (particularly around Philadelphia) or New Jersey.
- In Maine, Vermont and northern New Hampshire, it refers to any non-native, but particularly one from southern New England (including Massachusetts), downstate New York, or New Jersey and carries the additional connotation of someone who has recently moved to the area and would prefer that the state change to better accommodate newcomers, rather than the other way around. 
- In the Upper Peninsula of Michigan, it refers to someone from Wisconsin.
- In Northern Michigan, it refers to people from lower Michigan (those below Mt. Pleasant).
- In lower Michigan, it refers to people from Indiana or Ohio.
- In Wisconsin, it refers to someone from Illinois.
- In Georgia, it refers to someone from Florida.
- (physics) An inhabitant of or observer in a universe with two spatial dimensions.
1978, Henry Wesley Grayson, The theory of relativity revisited:
- To the flatlander the third dimension necessarily appears to be a process, something he travels through as he moves or is shifted across an area. He cannot occupy more than one position in the third dimension simultaneously.
- 1979, A Form of Pantomime, in Link, volume 21, part 3, page 86:
- The perceptual acts of the two-dimensional flatlander are seen by the projective Euclidean eyes as funny, [...]
1991, Floyd Merrell, Unthinking thinking: Jorge Luis Borges, mathematics, and the new physics, page 232:
- For our omniscient Mathematician, on the other hand, the time dimension from the beginning to the end of the game would be copresent, as would be our gaze of a flatlander's world.
2009, Frank Close, Nothing: a very short introduction, page 140:
- Earlier we gave the example of a plane taking off in the third dimension apparently disappearing from the view of a two-dimensional flatlander; analogously, particles appearing from the fifth dimension, or disappearing into it, could be a signal at the LHC that space-time is indeed, like Emmenthal cheese, permeated with little bubbles which are at the edge of our present abilities to measure.
- (cycling) A flatland BMX rider.