Ghost
Ghost turns up in 2 fields: photonics, fiber optics. Here is what it means to the people in each one.
Ghost in photonics
A weak stray beam from a back-surface reflection that travels off in an unexpected direction or focuses somewhere it should not.
A secondary reflection from a lens surface, window, filter or beamsplitter back side that carries a few percent or less of the main beam. Curved surfaces can focus a ghost to a tight spot inside another optic or on the operator's side of the table, and ghosts at a high-power laser can still exceed safe exposure. Good practice is to trace and block every ghost when building a setup.
"There's a ghost off the lens going straight up toward the ceiling, put a block above it and tilt the lens a degree."
Ghost in fiber optics
A false event on an OTDR trace caused by a strong reflection bouncing back and forth in the fiber.
An echo of a real reflective event that shows up on the trace where there is nothing in the fiber, usually at twice the distance of a strong reflection or out past the end of the fiber in the noise. A ghost shows no loss across it, which gives it away. Shorter pulses, a lower-reflectance far end, index-matching gel or cleaning the highly reflective connector make it disappear.
"That spike at 2 km on a 1 km cable is a ghost, it's sitting at exactly twice the panel distance."