The thing in your photo is a pop-up drain stopper (also commonly called a pop-up sink drain, pop-up assembly, or push/pop-up lavatory drain).
More specifically:
- The visible top part with the black dome-shaped cap is the stopper head, cap, or push-button cap — it's the part you press down on to open/close the drain.
- The shiny chrome cylindrical portion below it (with the threaded shaft) is the plunger or stopper body that moves up and down inside the drain pipe to seal or release water.
- The white ring is likely a gasket or mounting flange (often plastic/rubber) that seals it against the sink basin.