What is a Unionized Word™?

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 a Unionized Word™.

Use the examples below to find the rule.

$$

% 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{ Unionized }}

%

\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.2017.02.04}\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{ Akin }&\text{ Alike }\\ \hline

\text{ Aria }&\text{ Solo }\\ \hline

\text{ Cook }&\text{ Bake }\\ \hline

\text{