TITLE: What is the symbol ≙ most commonly used for in a mathematical or math-related context?
QUESTION [8 upvotes]: What is the symbol ≙ most commonly used for in a mathematical or math-related context?

LaTeX produces the symbol with \hateq.
The symbol has Unicode codepoint U+2259.
The respective Unicode code chart shows it with the name ESTIMATES and the annotation = corresponds to. Note that the names and annotations there are often not helpful, incomplete, or even misleading.
Entering it into the English Wikipedia currently redirects to the article Correspondence (mathematics).

Where and how is ≙ used in math or related fields? (By "related" I am thinking of statistics and physics.)
I believe to remember that I have seen it in some sort of "correspondence" meaning in a college-level science class.

REPLY [6 votes]: The hat above the equals sign is an estimator. Originaly estimators are used on terms. For example:
$$\widehat{\theta}(X)$$ 
But it has appeared (sometimes) more convenient to use it with an equals sign, especially in statistics. It has transformed a substantive (θ is an estimator for . . .) into a verb.
x ≙ . . . then reads as x estimates . . .
You use it instead of a plain equals sign when you do not have a strict equality between the  left term and the right expression. Statistically speaking, it symbolizes an estimation of an unobservable quantity.