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A lifter designed to collapse on command, which has stopped collapsing or stopped re-locking afterwards. This is the expensive failure behind cylinder deactivation codes, and it is common enough to be worth naming.

It is a more complicated part than an ordinary hydraulic lifter. A normal lifter takes up valve clearance and nothing else. A deactivating one carries a locking pin that oil pressure pushes aside, letting the body telescope so the camshaft lobe pushes into nothing and the valve stays shut. The pin has to release cleanly, and then re-lock cleanly, thousands of times.

The failure that matters is the re-lock. A pin that does not seat leaves the valve out of step with a camshaft still turning against it — the cylinder misfires, and the lobe riding an incorrectly loaded lifter can wear flat. At that point the repair is a camshaft and a full set of lifters rather than a solenoid, which is why a deactivation code accompanied by a misfire or a load-dependent tick is a reason to stop driving rather than to clear it and watch.

Oil is what actuates the pin, so oil level, grade and service history are diagnostic questions here and not maintenance ones. Many of these failures are the end of a long run of neglected oil changes.