It's the client who defines the project requirements. The project requirements are then shaped by the project manager based on the feedback from his team (as there are things that might not be technically feasible in the original requirements). The project managers will then get the final requirements approved by the client in order to approve the work.
For internal projects, it is the customer (who might be an entity in the organization, and not just and individual) who defines the requirements. Again, the requirements are shaped based on feedback from the project team and they are then approved by the customer.
All change requests are originally initiated by the client or the customer. Note that both the client and the customer are considered to be stakeholders.