inner world; one's world within
Refers to a person's internal psychological or emotional landscape, as opposed to the external world. Often used in literary or psychological contexts.
His novel deeply depicts the inner worlds of its characters.
Through meditation, I face my own inner world.
内面 means 'inner side' or 'inner self' and is more common in everyday use. 内面世界 is a more specific compound emphasizing the entire inner world as a realm.
外界 means 'the external world' or 'the outside world,' often used in contrast to 内面世界.
Compound of 内面 (ないめん, 'inner side/self') and 世界 (せかい, 'world'). The term is a straightforward combination of these elements.