[dspoka]

what does virtual mean in this case? is this word mean something in quantum physics?

[FiatLuxDave]

Virtual means that the particle does not obey the Einstein energy-momentum equation: E^2 = p^2c^2 + m^2c^4 . You might recognize the shorter form, where the particle has zero momentum, as E=mc^2.

More info here: https://en.wikipedia.org/wiki/On_shell_and_off_shell