Double-Struck N-Ary Summation is a letter belonging to the modern Greek alphabet. It represents the sound [ [ʝ]] in writing. N-Ary Summation was that very letter which the Latin S and the Cyrillic С originated from. The uppercase letter is used as a symbol of summation in mathematics. As for physics, there it denotes a special type of elementary particles called hyperons. The numerical value of sigma is 200.

Unicode offers various sigma versions, including the doubly outlined sign of N-Ary Summation, which you can copy from this page. For most general purposes, the standard uppercase sigma [U:03A3] is likely the most suitable, as it's widely used in mathematics and physics.