in logic, the mean or middle term of a syllogism, being an argument, reason, or consideration, for which we affirm or deny any thing; or, it is the cause why the greater extreme is affirmed or denied of the less in the conclusion.

arithmetic, or arithmetical medium or mean, called in the schools medium rei; that which is equally distant from each extreme, or which exceeds the lesser extreme as much as it is exceeded by the greater, in respect of quantity, not of proportion; thus 9 is a medium between 6 and 12.

Geometrical Medium, called in the schools medium persona, is that where the same ratio is preserved between the first and second as between the second and third terms; or that which exceeds in the same ratio or quota of itself, as it is exceeded: thus 6 is a geometrical medium between 4 and 9.

in philosophy, that space or region through which a body in motion passes to any point; thus ether is supposed to be the medium through which the heavenly bodies move; air, the medium wherein bodies move near our earth; water, the medium wherein fishes live and move; and glass is also a medium of light, as it affords it a free passage. That density or consistancy in the parts of the medium, whereby the motion of bodies in it is retarded, is called the resistance of the medium; which, together with the force of gravity, is the cause of the cessation of the motion of projectiles.

Subtile or Ætherial Medium. See Ether, Electricity, Fire, &c.

in optics, any substance through which light is transmitted.