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I came across this new symbol while reading a document about writing proofs, and I have never seen it before.

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    $\begingroup$ I've only seen it used to mean that the LHS is defined to be the RHS. $\endgroup$ – Eric Stucky Oct 11 '13 at 19:03
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    $\begingroup$ It is the eject button. $\endgroup$ – copper.hat Oct 11 '13 at 19:19
  • $\begingroup$ @copper.hat, awesome!! $\endgroup$ – Eleven-Eleven Nov 29 '13 at 1:32
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It’s is defined to be equal to; it’s my preferred symbol, but the most common one is $:=$, and I’ve also seen $\overset{\text{def}}=$.

For each $x\in X$ there is an open nbhd $U_x$ of $x$ such that ... . Then $\mathscr{U}\triangleq\{U_x:x\in X\}$ is ...

The $\triangleq$ indicates that $\mathscr{U}$ is being defined to be $\{U_x:x\in X\}$: we are not saying that some previously defined $\mathscr{U}$ is equal to the collection of these sets $U_x$.

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    $\begingroup$ +1, except that I don't agree with preferring this symbol ;) $\endgroup$ – Hagen von Eitzen Oct 11 '13 at 20:34
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    $\begingroup$ The $\overset{\text{def}}=$ symbol is better exactly because it doesn't lead to this question being asked as often... But the triangle-equals looks neater inline, is faster to write by hand, and is language-independent. So they're evenly matched. So let's invent a new symbol, how about $\overset{\leftrightarrow}=$ implying that LHS is a shorthand for RHS. $\endgroup$ – Evgeni Sergeev Nov 22 '13 at 3:22
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    $\begingroup$ +1 only because it is Brian's preferred symbol and that is a big deal! $\endgroup$ – ILoveMath Dec 7 '13 at 8:06
  • $\begingroup$ the best is :=. $\endgroup$ – user111072 Dec 30 '13 at 17:35
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    $\begingroup$ Some people might prefer $\triangleq$ because it is symmetrical. $\endgroup$ – Karlo Nov 26 '16 at 0:06
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The symbol means it is "equal to by definition".

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-4
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It actually is defined as "estimates", and also means that the content on the two sides correlates or conforms to each other.

For further reference, see here and here, the symbol is quite often used in (German?) academic context.

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    $\begingroup$ You are mistaking $\overset{\scriptscriptstyle\wedge}{=}$ for $\triangleq$. $\endgroup$ – emma Jun 5 '18 at 10:11

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