(1) A virtual X (where X is a common noun) is something, not an X, which has the efficiency (virtus) of an X.
This is the proper meaning of the word; but (2) it has been seriously confounded with ‘potential,’ which is almost its contrary. For the potential X is of the nature of X, but is without actual efficiency. A virtual velocity is something not a velocity, but a displacement; but equivalent to a velocity in the formula, ‘what is gained in velocity is lost in power.’
So virtual representation was the non-representation of the American colonies in the British Parliament, which was supposed to be replaced by something. So Milton asks whether the angels have virtual or immediate touch. So, too, the sun was said to be virtualiter on earth, that is, in its efficiency.
(3) Virtual is sometimes used to mean pertaining to virtue in the sense of an ethical habit.