If a word conforms to a special rule, I call it an Outside Word™.
Use the examples below to find the rule. I have made a mistake with a few of the words, so I've marked them with an "(E)" for error, while leaving them in so as not to invalidate Deusovi's correct answer.
|Not Outside Words™
And, if you want to analyze, here is a CSV version:
Outside Words™,Not Outside Words™
The puzzle satisfies the series' inbuilt assumption, that each word can be tested for whether it is a Outside Word™ without relying on the other words.
These are not the only examples of Outside Words™; many more exist.
What is the special rule these words conform to?