Plain English Meaning
The altimeter needle stops moving and keeps showing the same altitude, even though the aircraft is actually climbing or descending. It looks like it is working, but it is not.
Full Definition
An altimeter whose indication is stuck at a fixed value because its static port is blocked, preventing the instrument from sensing changes in outside air pressure as the aircraft climbs or descends.
Context Anchor
Encountered in pitot-static system discussions, especially when studying what happens after a blocked static port.
Derivation
Frozen' here is used in the sense of 'locked in place' or 'unable to move,' not necessarily caused by ice (though ice is one common cause). The instrument is figuratively frozen at a single reading.