摩訶止觀 5.82

5.82

○第一、觀陰入界境者，謂：五陰、十二入、十八界也。陰者，陰蓋善法，此就因得名。又陰是積聚，生死重沓，此就果得名。入者，涉入，亦名輪門。界名界別，亦名性分。

First, as for contemplating the object that is the aggregates, sense fields, and realms: this refers to the five aggregates, twelve sense fields, and eighteen realms. 'Aggregates' (skandha) conceal wholesome dharmas; this is named from the perspective of causes. Furthermore, 'aggregates' means accumulation, the layered accumulation of birth and death; this is named from the perspective of effects. 'Sense fields' (āyatana) means entering into involvement; it is also called the wheel-gate. 'Realms' (dhātu) means distinct realms; it is also called essential natures.