The LaTeX command for triangleq (≜) is \triangleq (defined as / triangle over equals).
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Write \triangleq inside math mode, that is between $…$ for inline math or in a \[ … \] or equation block for display math. This symbol needs the amssymb package, so add \usepackage{amssymb} to your preamble.
As a relation, \triangleq gets a thick space on each side, and a long inline formula may break after it. With unicode-math under XeLaTeX or LuaLaTeX, typing ≜ in a formula gives the same symbol.
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Use \triangleq in math mode after adding \usepackage{amssymb} to your preamble. For example, $a \triangleq b$.
It comes from the amssymb package: add \usepackage{amssymb} to your preamble.