The LaTeX command for curl is \nabla \times (curl / rotation).
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Write \nabla \times inside math mode, that is between $…$ for inline math or in a \[ … \] or equation block for display math. It is part of core LaTeX math, so no extra package is required.
The physics and siunitx packages offer shortcuts for many formulas, but \nabla \times works in plain LaTeX.
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^\circ\mathrm{C}\infty\dagger\hbar\nabla\nabla^2\nabla \cdot\partial\dot{x}\ddot{x}\langle \psi || \psi \rangle\langle \phi | \psi \rangle\rightleftharpoonsPhysics symbols in LaTeX: the full table or search all 350+ LaTeX symbols.
Use \nabla \times in math mode; it needs no extra package. For example, $\nabla \times$.
None: \nabla \times is built into LaTeX, so no extra package is required.