Also called: ctid, tuple identifier, item pointer
In plain English
A TID is the physical address of a row version: a pair of numbers written (block, slot). The first is which page the row lives on, the second is which line-pointer slot on that page. The system column ctid exposes it on every table.
Why it matters
Indexes store TIDs to find heap rows, and an UPDATE sets the old version’s t_ctid to point at the new version — that pointer is what forms a HOT chain. A TID is not stable: VACUUM and updates move rows, so never store a ctid as a long-lived key. It is, however, perfect for inspecting a specific page with pageinspect.