Plain English Meaning
It is the same kind of instrument as a regular vertical speed indicator, but it shows your climb or descent rate right away instead of taking several seconds to catch up.
Full Definition
An instantaneous vertical speed indicator is a vertical speed indicator that uses an internal accelerometer-driven pump to sense vertical acceleration and produce an immediate, accurate display of climb or descent rate, eliminating the lag that is normal in a standard VSI.
Context Anchor
Seen on the instrument panel during instrument flying, especially when checking whether a pitch or power change has started a climb or .