Tile
A tile is the smallest piece of the world-map. The whole factorio world is made out of them.
Because no other measurement is available, tiles are the only way to measure distances. Some examples:
Area covered by radar: 100x100 tile square. Speed of basic belt: about 1.8 tiles/sec. A Chunk has 32x32 tiles.
Tile is generally assumed to be 1 square meter in size (like Minecraft).