In conventional geodetic cartography, a grid is a two-dimensional orthogonal cartesian system used to plot a graticule where the projection belongs to the aphylactic class - the projection is neither conformal/orthomorphic nor authalic/equal-area. For instance, the cartesian system associated with the Cassini-Soldner projection is a grid. The cartesian system associated with the Polyconic projection is a grid. The cartesian system associated with the Polyhedric/Polyeder projection is a grid.
Since World War II, the term has been used interchangably with conformal projections also. Grid is now commonly used with reference to any cartesian system, regardless of the properties/class of the projection.