In the spirit of the What is a Word™/Phrase™ series started by JLee, a special brand of Phrase™ and Word™ puzzles.
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.
$$ % set Title text. (spaces around the text ARE important; do not remove.) % increase Pad value only if your entries are longer than the title bar. % \def\Pad{\P{0.0}} \def\Title{\textbf{ Outside }} % \def\S#1#2{\Space{#1}{20px}{#2px}}\def\P#1{\V{#1em}}\def\V#1{\S{#1}{9}} \def\T{\Title\textbf{Words}^{\;\!™}\Pad}\def\NT{\Pad\textbf{Not}\T\ }\displaystyle \smash{\lower{29px}\bbox[yellow]{\phantom{\rlap{rubio.2019.05.15}\S{6px}{0} \begin{array}{cc}\Pad\T&\NT\\\end{array}}}}\atop\def\V#1{\S{#1}{5}} \begin{array}{|c|c|}\hline\Pad\T&\NT\\\hline % \text{ FEED }&\text{ STARVE }\\ \hline \text{ HEN }&\text{ ROOSTER }\\ \hline \text{ DIRE }&\text{ TIRE }\\ \hline \text{ SEND }&\text{ SENT }\\ \hline \text{ SQUISH(E) }&\text{ SQUASH }\\ \hline \text{ GREEN }&\text{ BLUE }\\ \hline \text{ HEX }&\text{ CURSE }\\ \hline \text{ WIND }&\text{ WAND }\\ \hline \text{ SEEDS }&\text{ SESAME }\\ \hline \text{ RIDDLE(E) }&\text{ PUZZLE }\\ \hline \text{ INSIDE }&\text{ OUTSIDE }\\ \hline \text{ SHORE }&\text{ BEACH }\\ \hline \text{ OZ }&\text{ WIZARD }\\ \hline \text{ DEODORIZE }&\text{ STINK }\\ \hline \text{ HOW }&\text{ TO }\\ \hline \text{ PROGRAM(E) }&\text{ CODE }\\ \hline \text{ REP }&\text{ REPUTATION }\\ \hline \end{array}$$
And, if you want to analyze, here is a CSV version:
Outside Words™,Not Outside Words™
FEED,STARVE
HEN,ROOSTER
DIRE,TIRE
SEND,SENT
SQUISH(E),SQUASH
GREEN,BLUE
HEX,CURSE
WIND,WAND
SEEDS,SESAME
RIDDLE(E),PUZZLE
INSIDE,OUTSIDE
SHORE,BEACH
OZ,WIZARD
DEODORIZE,STINK
HOW,TO
PROGRAM(E),CODE
REP,REPUTATION
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?