Definitions:
almost or nearly as described, but not completely or according to strict definition

not physically existing as such but made by software to appear to do so

relating to the points at which rays would meet if produced backwards.

relating to or denoting infinitesimal displacements of a point in a system.

denoting particles or interactions with extremely short lifetimes and (owing to the uncertainty principle) indefinitely great energies, postulated as intermediates in some processes.