Final answer:
The department manager was using lateral communication to discuss a cost savings idea with managers in other departments.
Step-by-step explanation:
The communication channel that the manager was using to discuss a cost savings idea with other managers in different departments is lateral communication.
Lateral communication refers to communication that occurs between individuals or departments at the same level in the organizational hierarchy. In this case, the department manager is reaching out to managers in other departments who have similar levels of authority within the organization.
This form of communication allows for collaboration, exchange of ideas, and coordination across departments, benefiting all parties involved.