comitative

American Heritage® Dictionary of the English Language, Fourth Edition

1. adj. Of, relating to, or being the grammatical case indicating accompaniment in some languages, as in Finnish professorine "(along) with the professor.”
2. n. The comitative case.
3. n. A word or form in the comitative case.

Century Dictionary and Cyclopedia

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

Wiktionary

1. adj. Of, or relating to the grammatical case that is used in some languages to indicate accompaniment.
2. n. The comitative case, or a word in this case.