The American Heritage® Dictionary of the English Language, 4th Edition
adj. That can be tuned: a tunable wind instrument; a tunable radio.
adj. Archaic Tuneful.
the GNU version of the Collaborative International Dictionary of English
adj. Capable of being tuned, or made harmonious; hence, harmonious; musical; tuneful.
The Century Dictionary and Cyclopedia
Capable of being put in tune, or made harmonious.
Harmonious; musical; tuneful.
Word Usage
"But bending light so that it passes round a region of space and emerges travelling along the same line as it was initially is a difficult trick, requiring an invisibility cloak made from materials with a 'tunable' refractive index."