A person who requests a service or task, especially in professional contexts such as law, business, or investigation.
The lawyer listened carefully to the client's story.
The detective received an investigation request from the client.
クライアント is a common loanword for 'client' in business and IT, often interchangeable with 依頼者 but more casual and broader in scope.
Compound of 依頼 (irai, 'request') + 者 (sha, 'person'). The reading is standard on'yomi.