Δ means upper case delta .
Upper-case delta (Δ) often means "change" or "the change in" in mathematics. For example, if the variable "x" stands for the movement of an object, then "Δx" means "the change in movement." Scientists use this mathematical meaning of delta often in physics, chemistry, and engineering, and it appears often in word problem.
ex: initial movement P1 and final movement P2
So change of movement =Δp=p2- p1