GA4 property
The container in Google Analytics 4 that holds a set of data streams and the configuration applied to them.
What makes it wrong.
The mechanism, not the warning.
Data retention is a property setting with a short default, and it applies to the event-level data behind custom explorations, not to the standard aggregated reports. A question that needs event-level history often cannot be answered at all beyond the retention horizon, and the interface answers with an empty result rather than an explanation.
In detail.
A property is the unit of reporting, of retention settings and of access. Several sites and apps can feed one property through separate data streams, which is what makes cross-platform reporting possible and also what makes a property's numbers depend on which streams are attached.
The property identifier, not the account, is what every reporting call takes.
Where this is measured.
GA4 property comes from Google Analytics 4. The server page lists every tool that returns it, with its parameters.