A common word for a rectangle, emphasizing the four corners. Often used in everyday contexts like describing shapes of objects or rooms.
This room is shaped like a rectangle.
Please fold the rectangular paper in half.
Square, a special case of rectangle with all sides equal. 長四角 implies unequal adjacent sides.
Compound of 長 (naga, 'long') and 四角 (shikaku, 'quadrilateral' or 'square'), literally 'long quadrilateral'. The reading ながしかく is a straightforward combination of the kun'yomi of 長 and the on'yomi of 四角.