Other forms: rectangularly
In geometry, something that is rectangular had four sides and four right angles. Cereal boxes, dollar bills, and many cell phones have a rectangular shape.
Anything shaped like a rectangle is rectangular. Your backyard may be rectangular, and your brother's oversized suit jacket might give his body a somewhat rectangular shape. The most important aspect of something that makes it rectangular is those four right angles, and the word itself is rooted in the Latin rectus, "right," and the Old French angle, "angle."