Mathematical Monospace Small D

It is a distinct mathematical alphabet form of d.

Monospace means each character is designed to take the same width, like letters on a typewriter. In a formula, mathematical letter styles can distinguish symbols: 𝚍 can stand for a variable, a letter representing a value, different from plain d; the formula tells you what that variable represents.

Mathematical printing used styled letters before Unicode encoded them as separate characters. This family gives those distinctions a text form for mathematical and technical notation. Outside a formula, 𝚍 still gives the letter a typewriter-like appearance; the character records a style, not a fixed drawing, so the font supplies its visible shape.