A more thorough explanation:

Concerted refusal to deal is an agreement between two or more individuals or companies to not do business with a third party. This can be done by competitors or non-competitors.

For example, if two companies agree to not purchase goods from a particular supplier, this would be considered a concerted refusal to deal. Another example could be if a group of employees agree to not work for a certain employer.

Concerted refusals to deal can be illegal under the Sherman Antitrust Act. Depending on the nature of the agreement, it may be analyzed under the per se rule or the rule of reason.