suppositive

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

• adjective Of the nature of, including, or involving supposition.
• noun A conjunction, such as if or providing, that introduces a supposition.

from The Century Dictionary.

• Supposed; including or implying supposition.
• noun A conditional or continuative conjunction, as if, granted, provided.

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

• adjective Including or implying supposition, or hypothesis; supposed.
• noun A word denoting or implying supposition, as the words if, granting, provided, etc.

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

• noun A word denoting or implying supposition.