pellman
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Looks like \triangleq. I first came across it on the wikipedia article for Lie derivative. http://en.wikipedia.org/wiki/Lie_derivative#The_Lie_derivative_of_a_function
The discussion centers around the meaning of the triangle equals sign (\triangleq) and its distinction from the triple equals sign (\equiv). Participants explore its usage in mathematical contexts, particularly in relation to definitions and identities.
Participants generally agree on the definition of \triangleq as a symbol for "is defined as," but there is a question regarding its distinction from \equiv, indicating that the discussion remains unresolved on this point.
The discussion does not clarify the precise mathematical contexts in which each symbol is used, nor does it address potential variations in definitions across different fields.