The term comes from pharmaceutical storage but applies equally to food. If vaccines meant for +2 to +8 °C reach +12 °C during a power cut, or fall to −1 °C because a refrigerator was set too cold, that is an excursion. Each one must be detected, recorded, assessed for impact on the product and acted on.
Good practice is to document what happened, how high or low the temperature went, for how long, which products were affected and what was done. Whether a product can still be used is decided according to the manufacturer's stability information and the organisation's or programme's procedures, not by guesswork. Recurring excursions point to a root cause, such as a failing unit, poor placement, overloading, door habits or unreliable power, that needs fixing.
A hospital pharmacy in Chennai whose data logger shows a refrigerator reached +11 °C for forty minutes overnight would quarantine the affected stock, record the event, consult the product information and its standard procedures, and then review why the unit warmed.