James
Gregory (1636-1675) in his work “ Vera Circuli et Hyperbolae Quadratura”
(1667) considered function as a quantity obtained from other quantities by
successive use of algebraic operations or by any other operations Later G W Leibnitz (1646-1716) in his manuscript “Methodus tangentium inversa, seu  de
functionibus” written in 1673 used the word ‘function’ to mean a quantity varying
from point to point on a curve such as the coordinates of a point on the curve, the
slope of the curve, the tangent and the normal to the curve at a point