Cell in Cellular communication
The theoretical coverage area of a given cellular (or other UHF) transmitter will be circular, if it uses an omni-directional antenna - but most of the cell sites I've seen have multiple directional antennas, so the actual coverage area may have a non-circular shape.  Buildings and other obstructions may also alter the shape of the coverage area.

When making a conceptual drawing of multiple cell cites, it is common to show each cell's coverage as a hexagon, as that results in the most compact "packing" of cell sites.  In practice, cell sites can't be placed in a perfect hexagonal pattern, but that pattern works well for general discussions of the cell system.