If a triangle has three sides, a, b, and c, semi-perimeter, s, and area of triangle A, then the semi-perimeter and area of the triangle are given as:

$\text{s}=\frac{\text{a}+\text{b}+\text{c}}{2}$

Area of triangle $=\sqrt{\text{s}(\text{s}-\text{a})(\text{s}-\text{b})(\text{s}-\text{c})}$

According to the definition, s defines the semi-perimeter.

Hence, 's' in Hero’s formula is the semi-perimeter of a triangle.