One who seeks help choosing what to do.
This definition focuses on the purpose of the relationship between a client and a consultant. Pay doesn’t come into it — you (a “consultant”) might help your life partner (the “client”) decide the next step for their career.
In the consulting relationship, the consultant must help the client choose; they must not decide on behalf of the client. If you take the decision away from the client, you change the relationship. You are no longer helping them choose. Instead, you are managing a problem they have brought you. They are a customer.