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

in Arithmetic, or arithmetical medium or mean, called in the schools medium rei, is 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.

Philosophy, that space or region throughout 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 in which bodies move near our earth; water, the medium in which fishes live and move; and glass is also a medium of light, as it affords it a free passage. That density or consistency in the parts of the medium, by which the motion of bodies in it becomes 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. Sir Isaac Newton considers it probable that, besides the particular aerial medium in which we live and breathe, there is another more universal one, which he calls an ethereal medium; vastly more rare, subtle, elastic, and active, than air, and freely permeating the pores and interstices of all other mediums, and diffusing itself throughout the whole of creation, and by the intervention of which it is that, according to him, most of the great phenomena of nature are produced.

Optics, signifies any substance through which light is transmitted.