Mathematical Italic Small S

In LaTeX, type s in math mode; for plain text, use 𝑠.

In LaTeX, $s$ typesets s in the formula's math italic; it does not necessarily insert the standalone Unicode character. In HTML text, 𝑠 emits 𝑠 directly.

Separate mathematical letters let plain text retain distinctions that can change a formula's meaning. Math italic is tuned for equations and subscripts; ordinary text italics can look similar without being the same mathematical character.

What does mathematical italic s mean?

It marks a variable; the formula defines which one.

By itself, 𝑠 has no fixed value or quantity; an author assigns it a role in the formula.

Math italic has a long print history. Earlier algebra used Roman abbreviations and capitals; Descartes's La Géométrie established lowercase italic letters for unknowns in 1637. Separate mathematical characters later let plain text retain these style distinctions because different styles can designate different mathematical objects.

Why can 𝑠 turn into s?

NFKC normalization can fold it to plain s.

Some software normalizes text so compatibility forms compare alike. NFKC maps mathematical alphanumeric letters to their base Latin or Greek letters, so 𝑠 can become s and lose its mathematical style. Keep the character or use math markup when that distinction matters.