What does an inverted delta mean

The upside down capital delta is called a Del, or Nabla from the Greek νάβλα, meaning “harp” due to the shape. (Because of the story[1], I like “nabla” better.) According to Wolfram MathWorld[2] this symbol is used to indicate gradient or other vector derivatives.

The inverted Delta symbol and arrow of is called the “Del Operator.” Many texts will omit the vector arrow, which is also a faster way of writing the symbol. But the vector arrow is helpful to remind you that the gradient of a function produces a vector.