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.

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

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

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