Use for computational-device notional particles which are not observable in the lab, such as "off-the-mass-shell" particles, "spurions", renormalons, merons. Do not use for unobserved hypothetical particles such as the graviton, or Goldstone bosons which could have been observable, but got rearranged into other particles through QFT mechanisms.
A virtual particle is a notional intermediate computational device, often in perturbation theory, whose mathematical form resembles that of actual, observable particles, except it relaxes crucial properties of such particles, such as mass; it is then said to be "off mass shell", that is, it violates the relativistic constraint between mass, momentum and energy. It may have negative energy, progress backwards in time, or travel faster than light, all the while preserving some basic physics laws like energy-momentum conservation.
By the end of the calculation, no virtual particles remain to be observed, e.g. in a collision or decay phenomenon: the virtual particles may be visualized as "short-lived" intermediate props in the calculation: the farther off the mass shell (the larger their debt in energy), the shorter their notional (virtual) lifetime.
