What is Company Union?

Legal Definition
A company or "yellow" union is a worker organization which is dominated or influenced by an employer, and is therefore not an independent trade union. Company unions are contrary to international labour law (see ILO Convention 98, article 2). They were outlawed in the United States by the 1935 National Labor Relations Act §8(a)(2), due to their use as agents for interference with independent unions. Company unions persist in many countries, particularly with authoritarian governments.

Some labor organizations are accused by rival unions of behaving like "company unions" if they are seen as having too close and cordial a relationship with the employer, even though they may be recognized in their respective jurisdictions as bona fide trade unions.
-- Wikipedia
Legal Definition
A union that is fake. The employer can prevent the organization of a real union. AKA house union.