enquiry Definition

1. An act or the process of asking for information.

Thesaurus: question, interrogation, query, questioning.
2. An investigation, especially a formal one.

Thesaurus: investigation, probe, hearing, examination, analysis.
Form: an inquiry into something (often)

Inquiry and enquiry are interchangeable variants in the more general sense 1. Inquiry and not enquiry is used in the more formal and specific sense 2.