Mathematical Italic Small D

It is an italic mathematical letter; the equation defines its role.

Think of it as the Latin letter d in a math-specific style. A formula can use it for a variable or quantity; it has no fixed value on its own.

This style grew from print notation, not from a one-time invention of a new d. Early printers used the type they had: Cardano’s Ars magna, an algebra book, used upright abbreviations for unknowns in 1545; Vieta used upright capitals in 1591. René Descartes’s work in 1637 helped establish lowercase italic letters for unknowns. The style distinguished symbols in formulas; Unicode later gave mathematical alphabets their own characters so plain text could retain those distinctions.

Can an equation show italic d without storing 𝑑?

Yes; equation markup can supply its mathematical style.

MathML, a web format for equations, can mark ordinary d as a mathematical identifier and render it in math italic. In that case, the equation stores d plus markup, not the literal 𝑑.

Fonts draw mathematical italics differently, and the renderer needs a font that supports the glyph. For plain text, use a Unicode character picker when you need this exact character; a rich-text italic button changes formatting only.