The square metre (also spelled square meter, see spelling differences) is the SI derived unit of area, with symbol m2(33A1 in Unicode). It is defined as the area of a square whose sides measure exactly one metre. The square metre is derived from the SI base unit of the metre, which in turn is defined as the length of the path travelled by light in absolute vacuum during a time interval of 1⁄299,792,458 of a second.