A tile is the smallest piece of the world map. The whole factorio world is made out of them.
Tiles are both: a unit to measure area and distance.
A Chunk is 32x32 tiles. A chunk is the next biggest unit for area/distance.
- Area covered by radar: 100 tiles Diameter (100 tiles in each direction)
- Speed of a basic belt: about 1.8 tiles/sec.
- It takes about 2 minutes and 47 seconds to run 1000 tiles without speed up. Which means that the character is a very healthy man/woman.
Real size of tiles
1 Tile is generally assumed to be 1 square meter in size (like Minecraft).
Length or area?
If you want to make it clear whether you are referring to a tile as length or area please use something like "tile length", "square tiles", "tile^2" or similar, but in most cases it should be clear.