tenant isolation
The rule that keeps one customer's data invisible to every other customer on the same service.
Most online services keep all their customers in one shared database. A tenant is one customer, and tenant isolation is the set of rules that makes sure your rows are only ever returned to you. Get it right and nobody notices. Get it wrong on a single table and every account on the platform becomes readable by anyone who signs up.
It is one of the most common serious flaws in modern apps, partly because it is invisible in normal use: the product works perfectly for honest users while quietly answering questions it should refuse. Compliance certificates describe process rather than the actual state of a database, which is why they routinely fail to catch it.