Platform Contract
The platform's promise to its consumers about interface stability, behaviour and support, treated with the same seriousness as an external API.
Internal consumers cannot choose another supplier, which teams sometimes read as permission to change things freely. It is the opposite: because they cannot leave, a breaking change imposes unbudgeted work on every one of them simultaneously, and the platform team does not bear that cost.
A platform contract states what is stable and for how long: which interfaces are supported, what compatibility guarantee applies, what notice a breaking change carries, what support response consumers can expect, and — importantly — what is explicitly not stable, so experimental surfaces can move quickly without becoming accidental commitments.
The marker of a mature platform is that this document exists and is honoured even when honouring it is inconvenient. The marker of an immature one is a changelog entry reading "the field was renamed for clarity", which is a breaking change described as an improvement, and which teaches every consumer to pin versions and never upgrade.