In electricity, in alternating currents, effective: said of the value which is to be used in computing energy or power relations of a current.In synchronous alternating-current machines, the induced electromotive force corresponding to the resultant of the magnetomotive forces of field-flux and armature-flux.Existing in effect, power, or virtue, but not actually: opposed to real, actual, formal, immediate, literal.Pertaining to a real force or virtue; potential.In mech., as usually understood, possible and infinitesimal: but this meaning seems to have arisen from a misunderstanding of the original phrase virtual velocity, first used by John Bernoulli, January 26th, 1717, which was not clearly defined as a volocity at all, but rather as an infinitesimal displacement of the point of application of a force resolved in the direction of that force.