People who live in or are from London.
From the place name 'London' (from Roman 'Londinium,' possibly from a Celtic word) plus '-er' to denote inhabitants, then pluralized. This is a standard pattern for forming demonyms in English.
The way English names people from places is oddly unsystematic—Londoners, New Yorkers, Parisians, Romans—we inherited different naming patterns from different languages, showing how haphazard language evolution really is.
Complete word intelligence in one call. Free tier — 50 lookups/day.