Short for the gradient of a quantity. In an orthogonal coordinate system with coordinates {u1,u2,u3}, where a line element ds is given by

ds²=h1² du1² + h2² du2² + h3² du3²
then the gradient of a scalar quantity φ is given by
grad φ = (1/h1)(δ φ/δ u1) i1 + (1/h2)(δ φ/δ u2) i2 +(1/h3)(δ φ/δ u3) i3
where i is the unit vector

In cartesian coordinates

{u1,u2,u3} = {x,y,z}
{h1,h2,h3} = {1,1,1}

'Grad' is usually represented by an triangle 'pointing' downwards

See also div.