The following is an explanation copied from Viki encyclopedia:

Splitting operator and inverted triangle operator (nabla) are symbols. What is their shape? . The name comes from a Greek harp: Nabura. Related words also exist in Aramaic and Hebrew.

Another common name for this symbol is atled, because it is the inverted shape of the Greek letter δ. Besides atled, it also has a name called del.

Split operator is used to refer to gradient operator in mathematics. It is also used to refer to connections in differential geometry (in a broader sense, it can be regarded as gradient operators). It was put forward by Hamilton.

Besides, it's here, so I won't copy it:

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