\Vir"tu*al\ (?; 135), a. [Cf. F. virtuel. See {Virtue}.]
1. Having the power of acting or of invisible efficacy
   without the agency of the material or sensible part;
   potential; energizing.
         Heat and cold have a virtual transition, without
         communication of substance.           --Bacon.
         Every kind that lives, Fomented by his virtual
         power, and warmed.                    --Milton.
2. Being in essence or effect, not in fact; as, the virtual
   presence of a man in his agent or substitute.
         A thing has a virtual existence when it has all the
         conditions necessary to its actual existence.
                                               --Fleming.
         To mask by slight differences in the manners a
         virtual identity in the substance.    --De Quincey.
{Principle of virtual velocities} (Mech.), the law that when
   several forces are in equilibrium, the algebraic sum of
   their virtual moments is equal to zero.
{Virtual focus} (Opt.), the point from which rays, having
   been rendered divergent by reflection of refraction,
   appear to issue; the point at which converging rays would
   meet if not reflected or refracted before they reach it.
{Virtual image}. (Optics) See under {Image}.
{Virtual moment} (of a force) (Mech.), the product of the
   intensity of the force multiplied by the virtual velocity
   of its point of application; -- sometimes called {virtual
   work}.
{Virtual velocity} (Mech.), a minute hypothetical
   displacement, assumed in analysis to facilitate the
   investigation of statical problems. With respect to any
   given force of a number of forces holding a material
   system in equilibrium, it is the projection, upon the
   direction of the force, of a line joining its point of
   application with a new position of that point indefinitely
   near to the first, to which the point is conceived to have
   been moved, without disturbing the equilibrium of the
   system, or the connections of its parts with each other.
   Strictly speaking, it is not a velocity but a length.
{Virtual work}. (Mech.) See {Virtual moment}, above.