Customer implies consumerism whereas client implies an ongoing professional relationship with all that it implies; trust, quality, professionalism, partnership.
The client
The client can be a person, a management team, a whole department or even a superior.
Consultants should try to meet with each person in a client role at least once before initiating an intervention, even if they are at a very high level in the organisation.
This will allow the consultant to obtain focused information on the client’s wants and needs and whether what is being planned will satisfy them.