instrumental

American Heritage® Dictionary of the English Language, Fourth Edition

1. adj. Serving as a means or agency; implemental: was instrumental in solving the crime.
2. adj. Of, relating to, or accomplished with an instrument or tool.
3. adj. Music Performed on or written for an instrument.
4. adj. Grammar Of, relating to, or being the case used typically to express means, agency, or accompaniment.
5. adj. Of or relating to instrumentalism.
6. n. Grammar The instrumental case.
7. n. A word or form in the instrumental case.
8. n. Music A composition for one or more instruments, usually without vocal accompaniment.

Century Dictionary and Cyclopedia

1. Of the nature of an instrument or tool; serving as an instrument or means; used or serving to promote or effect an object; helpful; serviceable: as, the press has been instrumental in enlarging the bounds of knowledge.
2. Pertaining to, made by, or prepared for instruments, especially musical instruments. Specifically, in music, noting a composition or a passage intended for instruments rather than for the voice, or in a style not germane to the voice: opposed to vocal.
3. In grammar, serving to indicate the instrument or means: applied to a case, as in Sanskrit, involving the notion of by or with. In Anglo-Saxon and other Teutonic tongues this case is merged, with a few exceptions, in the dative; In the Latin, with the ablative. Abbreviated inst. or instrumental
4. n. An instrument.
5. n. The instrumental case. Compare I., 3.

Wiktionary

1. adj. Acting as an instrument; serving as a means; contributing to promote; conductive; helpful; serviceable; essential or central.
2. adj. Pertaining to, made by, or prepared for, an instrument, especially a musical instrument; as, instrumental music, distinguished from vocal music.
3. adj. Applied to a case expressing means or agency—and is generally indicated in English by by or with with the objective; as, the instrumental case. This is found in Sanskrit as a separate case, but in Greek it was merged into the dative, and in Latin into the ablative. In Old English it was a separate case, but has disappeared, leaving only a few anomalous forms. It continues to be used in Slavic languages.
4. n. The instrumental case.
5. n. A composition without lyrics.

GNU Webster's 1913

1. adj. Acting as an instrument; serving as a means; contributing to promote; conductive; helpful; serviceable.
2. adj. Pertaining to, made by, or prepared for, an instrument, esp. a musical instrument.
3. adj. Applied to a case expressing means or agency. This is found in Sanskrit and Russian as a separate case, but in Greek it was merged into the dative, and in Latin into the ablative. In Old English it was a separate case, but has disappeared, leaving only a few anomalous forms.

WordNet 3.0

1. adj. relating to or designed for or performed on musical instruments
2. adj. serving or acting as a means or aid