Definition
A cockpit-controlled valve that allows the pitot-static instruments (altimeter, airspeed indicator, and vertical speed indicator) to draw static air pressure from an alternate source -- usually inside the cabin -- when the normal external static port is blocked or obstructed.
Plain English
A switch in the cockpit that lets the pilot feed the flight instruments air pressure from inside the cabin if the outside static port gets blocked, so the instruments keep working.
Context Anchor
Seen in blocked static system discussions and in aircraft that have an alternate static source control in the cockpit.
Derivation
Alternate' means a backup or substitute. 'Static' here refers to still, undisturbed air pressure (as opposed to ram air hitting the pitot tube). Together: a backup source of still-air pressure for the instruments.
Why Pilots Care
Restores usable altimeter and vertical speed indications during static-port blockage so the pilot can continue instrument flight without immediate loss of altitude reference.
Analogy
It is like using a backup doorway when the normal doorway is blocked. The instruments still get the air pressure they need, but from a different place.
Intuition Check
Do not read “static” as meaning the valve does not move. Here, static means still surrounding air pressure used by the instruments; alternate means a backup path to that pressure.
Example Sentence 1
When the altimeter froze at the same reading during the climb, the pilot suspected a blocked static port and opened the alternate static source valve.
Example Sentence 2
During the preflight check the pilot verified that the alternate static source valve moved freely and returned to the normal position.