adjective: (geometry) Having, or being in, a contrary order; said of a section of an oblique cone having a circular base made by a plane not parallel to the base, but so inclined to the axis that the section is a circle; applied also to two similar triangles when so placed as to have a common angle at the vertex, the opposite sides not being parallel.
adjective: (logic) Denoting the relation of opposition between the particular affirmative and particular negative. Of these both may be true and only one can be false.
noun: (logic) Either of a pair of propositions at least one of which must be true
