noun
square; square shape
Refers to a geometric square or something square-shaped. Often used in technical or descriptive contexts.
この部屋は平方形をしている。
This room is square-shaped.
平方形のテーブルを買った。
I bought a square table.
四角 is a more casual, everyday word for a square or rectangle; 平方形 is more formal and explicitly refers to a square.
Compound of 平方 (へいほう, 'square' as in area or exponent) and 形 (けい, 'shape'). The exact historical derivation is uncertain, but the word transparently means 'square shape'.