Outer, outward, outside, external, exterior mean being or placed without something.
Although in many cases interchangeable, they are more or less restricted in their applications and are therefore clearly distinguished in their implications.
Outer usually retains its comparative force, then applying to what is farther out from something described as inner or is farther than another thing from the center.
Outer is also applicable to what is definitely without as opposed to what is definitely within something, but in this sense the term rarely suggests spatial relations; thus, the outer man is the man as known in the flesh and as distinguished from the inner man, that is, the man as he really is in mind and soul; one's outer life is the part which is observable to one's fellows; the outer world is the world as known directly through the senses.
Outward may be used of spatial relations; when it is so used it commonly implies motion or direction away from, or the reverse of, what is inward. Like outer, the term is sometimes used of what is manifest to others in contrast with what is within and especially with what is spiritual or mental.
Outside usually implies a position on or a reference to the outer parts or surface of a thing. But outside, in extended use, applies especially to a person or thing that is beyond implied borders, bounds, or limits; thus, an outside influence is one not emanating from the particular society, group, or community in mind; the outside world is the world beyond the scope or interest (as of a family group, community, or set) or the confines of a place (as an institution, a town, city, or a state); an outside broker is one who is not a member of an exchange; outside work is work in the open air in contrast with inside work under cover (as in an office, factory, or store).
External and exterior are often used interchangeably without loss, for both come close in meaning to outside.
But external may be preferred when location or situation beyond or away from the thing under consideration is implied and exterior is often preferred when location or situation on the surface or on the outer limits of a thing is implied.
In addition, external sometimes comes close to superficial in implying mere appearance or semblance that has no relation or little relation to what the thing really is and exterior may, like outer and outward, apply to what shows or is made apparent.