These words refer to what lies below the surface, in the central or inside portion of something, or within its boundaries. Inner is the most general of these and has the widest range of meaning. It can have geographical pertinence: the inner regions of the island. It can refer to a side that faces in: the inner edge of the terrace. It can also indicate the enclosed portion of something: an inner room of the restaurant, available for private parties. In addition to these physical denotations, the word suggests covert or unconscious mental activity: signs of an inner turbulence of emotion; an inner ripeness for change.

Interior also suggests the geographical context: the arid interior of Australia. It has a special relevance for something: hidden or obscure, suggesting enclosure: an interior courtyard of the Florentine variety. And it refers, most strictly, to thought processes that are carried on without being spoken aloud: interior monologue.

The emphasis with internal is on something completely surrounded and covered from view: the internal organs of the body; suspecting internal injuries. It can also refer to mental activity, relating less to interior in this context than to inner , although its point is often a distinction between feeling emotions and expressing them: an internal uneasiness that at last became apparent in his nervous pacing.

Inward often refers to spiritual states: an inward rectitude of being. Sometimes its emphasis is on invisible as opposed to visible states: his outward behaviour testifying to an inward state of grace. On a more mundane level, the word may indicate movement towards interior regions: their inward journey up the river, tracing its course upstream through the jungle.