subcontrary

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A proposition related to another in such a way that both may be true, but both cannot be false.

from The Century Dictionary.

• Contrary in an inferior degree.
• noun In logic, a subcontrary proposition.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Logic) A subcontrary proposition; a proposition inferior or contrary in a lower degree.
• adjective Contrary in an inferior degree.
• adjective (Geom.) 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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun logic Either of a pair of propositions at least one of which must be true