slack-key

Etymology
From a translation of the.

Adjective

 * 1)  (of a string instrument or a manner of playing one) Performed with loosened strings, such that all of the strings play in the same key (most often G major).

Usage notes
Slack-key is almost always used in reference to Hawaiian music, where it is an important technique. In this context it may be referred to as a music genre, slack-key guitar. Transliterations of the original Hawaiian are also common: Kiluea tuning or ki ho'alu guitar.