首楞嚴義疏注經 3.165

3.165

梵云馱都，此云界，界是因義，根、境、識三互為因故。又種族義，根、境、識三各一種族。又眼等六，種族別故。

In Sanskrit it is called dhātu, translated here as realm (jie). 'Realm' has the meaning of 'cause', because the three—faculty, object, and consciousness—serve mutually as causes. It also has the meaning of 'category' (species/lineage), because faculty, object, and consciousness each constitute one category. Furthermore, the six beginning with the eye are distinct in their categories.