A "functional test" is.

A functional test is a dynamic test for which the tests are based on the specification of the test object. This means that the test cases and test data are derived from the functional requirements, use cases, user stories, or other sources that describe the expected behavior and functionality of the system under test. A functional test is not focused on the efficiency of the system (that would be a performance test), nor is it a static comparison of test and requirements specification (that would be a review or inspection), nor is it based on the structure of the test object (that would be a structural test).

References:

Certified Tester Foundation Level Syllabus - ISTQB Guru1, page 10

Certified Tester Foundation Level (CTFL) Syllabus - ASTQB2, page 65

A Study Guide to the ISTQB® Foundation Level 2018 Syllabus - Springer3, page 77