The LaTeX command for Laplacian is \nabla^2 (Laplace operator).
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Write \nabla^2 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^2 works in plain LaTeX.
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\rightleftharpoons\text{\AA}^\circ\mathrm{C}\infty\dagger\hbar\nabla\nabla \cdot\nabla \times\partial\dot{x}\ddot{x}\langle \psi || \psi \ranglePhysics symbols in LaTeX: the full table or search all 350+ LaTeX symbols.
Use \nabla^2 in math mode; it needs no extra package. For example, $\nabla^2$.
None: \nabla^2 is built into LaTeX, so no extra package is required.