noun
due south
Refers to the exact compass direction south, as opposed to a general southern area.
この部屋の窓は正南を向いている。
The window of this room faces due south.
南 is the general word for 'south' or 'southern area', while 正南 specifically means 'due south' (exact compass direction).
真南 also means 'due south' and is more common in everyday speech. 正南 is a more formal or technical term.
Compound of 正 (sei, 'correct' or 'exact') and 南 (nan, 'south').