What does an upside down triangle mean in maths notation?

Answer 1

The 'upside down' triangle symbol is the (greek?) letter Nabla. Nabla means the gradient. The gradient is the vector field whoose components are the partial derivatives of a function F given by (df/dx, df/dy).

Answer 2

In mathematics notation, an upside-down triangle typically represents the symbol for "change" or "difference." It is commonly used in calculus to denote the change in a variable or function. The upside-down triangle is known as the "nabla" symbol and is also used in vector calculus to represent the gradient of a scalar field.