In graphic design, a grid is a structure (usually two-dimensional) made up of a series of intersecting straight (vertical, horizontal, and angular) or curved guide lines used to structure content.

The Grid System - ArticlesThe Grid SystemJuly 5th, 2019
The designer's guide to grid theoryCreative BloqSeptember 10th, 2018
All About Grid SystemsEnvato Tuts+August 22nd, 2013
Fluid GridsA List ApartMarch 3rd, 2009
Thinking Outside the GridA List ApartDecember 19th, 2005