What is Object, image, stigmatism?

An optical system receives rays from a point $A$ (the object point). If all the emerging rays pass through one point $A'$, or all seem to come from one point $A'$, the system is stigmatic for $A$ and $A'$ is the image of $A$. The image is real if the emerging rays actually cross at $A'$ (it can be caught on a screen), virtual if only their backward extensions do (it is seen through the system, not projected). Likewise the object is real if the incoming rays diverge from $A$, virtual if they converge toward $A$ behind the system’s entrance. The pair $(A, A')$ is conjugate.