Wiktionary
long s
alt. The '''ſ''' character, as distinct from the short s — the '''s''' character. n. The '''ſ''' character, as distinct from the short s — the '''s''' character.
Wikipedia
Long s
The long, medial, or descending s (ſ) is a form of the minuscule letter s, which was formerly used where s occurred in the middle or at the beginning of a word (e.g. "ſinfulneſs" for "sinfulness" and "ſucceſsful" for "successful"). The modern letterform was called the terminal, round, or short s.
Usage examples of "long s".
Instead, he disappeared-but in his place was a glowing disk with a long S-curve down the middle.