What is Field-Level Security (FLS) and how is it distinct from page layouts?

Field-Level Security (FLS) is the authoritative security control that determines whether a user can see (visible) or edit (editable) a specific field, regardless of the page layout. It is configured via profiles and permission sets. Page Layouts control how fields appear on a record detail/edit page (position, required on layout, read-only on layout) for a specific profile/record-type combination. A field hidden via FLS will not appear anywhere — API, reports, Apex queries (in enforced contexts), or layouts. A field removed from a layout is still accessible via API and reports if FLS permits. FLS is security; page layout is presentation.