What is "dadgad"

DADGAD, or Celtic tuning is an alternative guitar tuning most associated with Celtic music, though it has also found use in rock, folk, metal and several other genres. Instead of the standard EADGBE tuning, the six guitar strings are tuned, from low to high, D2 A2 D3 G3 A3 D4. Tuning to DADGAD from standard is accomplished by tuning the first, second and sixth strings down a whole step (two frets). The result is an open D suspended fourth chord (see suspended chord). Being suspended, the open tuning is neither intrinsically major nor minor.