Plain English Meaning

A small hole made to an exact size so that air can only flow through it at a known, steady rate. It acts like a controlled leak that slows pressure changes down in a predictable way.

Full Definition

A precisely sized small opening engineered to allow air to pass through at a specific, controlled rate. In the Vertical Speed Indicator (VSI), a calibrated orifice creates a deliberate, metered leak between the instrument case and the static pressure source so that pressure inside the case lags behind outside pressure during a climb or descent, producing a measurable rate of change.

Context Anchor

Seen in discussions of how a vertical speed indicator works and why it has a slight delay when showing climbs or descents.