Ident
All mode A, C, and S transponders include an "IDENT" button, which activates a special thirteenth bit on the mode A reply known as IDENT, short for "identify"". When radar equipment receives the IDENT bit, it results in the aircraft's blip "blossoming" on the radar scope. This is often used by the controller to locate the aircraft amongst others by requesting the ident function from the pilot, e.g., "Cessna 123AB, squawk 0363 and ident".
Ident can also be used in case of a reported or suspected radio failure to determine if the failure is only one way and whether the pilot can still transmit or receive, but not both, e.g., "Cessna 123AB, if you read, squawk ident".
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