comitative

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

• adjective Of, relating to, or being the grammatical case indicating accompaniment in some languages, as in Finnish professorine “(along) with the professor.”
• noun The comitative case.
• noun A word or form in the comitative case.

from The Century Dictionary.

• That serves to express accompaniment, association, or connection; associative.
• In grammar, noting the case expressing companionship (with). Also as a noun.

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

• adjective grammar Of, or relating to the grammatical case that is used in some languages to indicate accompaniment.
• noun The comitative case, or a word in this case.