A natural key is what I would normally call unique index. Say first, surname and date of birth in an employee table as a bad (poor design) example. As opposed to a surrogate key like personId being an auto incrementing ID which is the norm since it can be easier to use when joining tables. I wish articles like these would explain terminology up front. I found it irritating to wade into the anecdotal trivia and not know what I was reading about and I’m very familiar with how databases work!