A grid is the finite viewable volume surrounding any object in space. This is typically a cube extending to approximately 8000 km in each cardinal direction (north, south, east, west, up, and down). In other words, if a ship is alone deep in space with nothing else visible on the overview, there will be an invisible cube surrounding that ship with an edge length of 16000 km. Anything uncloaked within this cube will be visible in the overview to all pilots within that same cube. An object located outside of the grid will not be visible in the overview.
Importantly, grids are not relative to the observer. In EVE Online's internal game logic, each grid has a fixed and definite boundary (sometimes called a "grid wall") that ships can pass through into the next grid. The game will create, expand, contract, and delete grids as needed depending on how objects (including players) move around in the solar system. Consequently, some grids are larger than the normal 8000 km distance: the grid around the Jita 4-4 trade hub extends about 20,000 km in each direction; all uncloaked objects within such grids are visible on overview to all pilots within the grid, despite being dozens of megameters away.
The game can not merge two or more adjacent pre-existing grids into a single grid, which can make grid walls more readily apparent.
Until 2016, the default grid size was 250 km in every direction from a central object. This allowed players to engage in “grid manipulation” (or “grid-fu”), using fast ships such as interceptors to push grids out in particular directions and thereby control their tactical environment, potentially concealing ships from nearby enemies. This was a complex, involved process, but was not considered an exploitation of the game mechanics. With the extension of the default grid size to 8000 km, grid manipulation is no longer possible for practical purposes.