A TPM 2.0 entity is an item in the TPM that can be directly referenced with a handle.
The term encompasses more than objects because the specification uses the word object to identify a very specific subset of entities. This can be confusing, so this chapter briefly describes all of the entity types: permanent entities (hierarchies, the dictionary attack lockout mechanism, and PCRs); nonvolatile entities (NVRAM indexes), which are similar to permanent entities; objects (keys and data); and volatile entities (sessions of various types).
After this introduction, the following chapters discuss each entity and its uses in more detail. In particular, the next chapter delves into hierarchies, a collection of entities that are related and managed as a group.
A permanent entity is one whose handle is defined by the TPM specification and can't be created or deleted. In TPM 1.2, PCRs and the owner were the only permanent entities; the storage root key (SRK) did have a fixed handle but wasn't a permanent entity. In TPM 2.0, there are more: three persistent hierarchies, the ephemeral hierarchy, the dictionary attack lockout reset, PCRs, reserved handles, the plaintext password authorization session, and the platform hierarchy NV enable. The following sections discuss each in turn.
TPM 2.0 has three persistent hierarchies (platform, storage, and endorsement), each referenced by a permanent handle: TPM_RH_PLATFORM (0x4000000C), TPM_RH_OWNER (0x40000001), and TPM_RH_ENDORSEMENT (x4000000B). Permission to use these hierarchies is granted through authorizations, so each has both an authorization value and a policy. Either can be changed at the will of the hierarchy's administrator (defined as anyone who can authorize such a change). The authorization value or policy value may change, but whenever we refer to, for example, the platform authorization, we mean the same entity. Persistent hierarchies can never be deleted, but they may be disabled by the administrator of the platform or the administrator of the hierarchy. These three hierarchies may have associated chains of keys and data, which can be wiped by clearing the hierarchy.
The next chapter describes the hierarchies in detail, including each hierarchy's management and use cases. At this point, it's sufficient to understand that the persistent hierarchies are permanent entities. They can't be created or deleted.
Other permanent entities similar to the hierarchies listed here are the ephemeral hierarchy and the dictionary attack lockout reset mechanism.