Virtual image
An image formed where reflected or refracted rays only appear to originate, because they are diverging when they reach the eye. No light reaches that location, so a virtual image cannot be projected on a screen.
Your eye traces diverging rays backwards and places an image where their extensions meet, behind a mirror or back on the object's side of a lens. Nothing is there. Put a card at the spot and it stays blank. The CED's phrasing is that the rays diverge such that they appear to have originated from a common point.
The sign test. A negative image distance from means virtual. That is the whole diagnosis.
What makes it the opposite of a [real image](/glossary/real-image) is that no light energy arrives. A real image can be caught, photographed, or landed on a sensor; a virtual one exists only for an observer looking back along the rays. For a single optic with a real object, a virtual image is also always upright, where a real one is always inverted.
Which optics make one. A plane mirror always, a convex mirror always, a diverging lens always, and a converging optic when the object sits closer than the focal point. That last case is the magnifying glass: upright, enlarged, on the same side as the print. Push the glass past and the image flips and turns real, which is the flip everyone has seen without naming.
Virtual does not mean small or fake. You see yourself in a bathroom mirror at full size, and that image is virtual. It only means the rays never met.