Definition of "q.e.d."

Now let any right line meet four harmonicals in A {\displaystyle A} , C {\displaystyle C} , B {\displaystyle B} , D {\displaystyle D} , and if thoſe harmonicals be parallel, the thing is evident; but, if they interſect in V, draw E C F {\displaystyle ECF} parallel to V D {\displaystyle VD} , the line moſt remote from C {\displaystyle C} ; then, by the firſt part, E C = C F {\displaystyle EC=CF} , and A D : A C :: V D : E C {\displaystyle AD:AC::VD:EC} or C F {\displaystyle CF} . Therefore A D : A C :: B D : C B {\displaystyle AD:AC::BD:CB} . q.e.d.

1803, “The Mathematical Repository, Nº 63”, in The Gentleman’s Diary, or The Mathematical Repository; an Almanack for the Year of Our Lord 1803: […], London: Printed for the Company of Stationers by Nichols and Son, […]; [a]nd sold by George Greenhill, […], page 33

Prop[osition] VI. One substance cannot be produced by another substance. Demonst[ration]. In the preceding proposition we have seen that there cannot in the nature of things be two Substances of the same attributes, or that they have anything in common (by Prop. II.); and so (by Prop. III.) one cannot be the cause of, or be produced by, another: q.e.d.

1870, Benedict de Spinoza [i.e., Baruch Spinoza], R[obert] Willis, “[The Ethics.] Part I.—Of God.”, in Benedict de Spinoza; His Life, Correspondence, and Ethics, London: Trübner & Co., […], page 418