Final answer:
The HSI SEL button is an aviation control that allows pilots to choose the source of navigational data for the Horizontal Situation Indicator in an airplane's cockpit.
Step-by-step explanation:
The HSI SEL button refers to the Horizontal Situation Indicator Selector button commonly found in aviation. This button is used in the cockpit of an airplane and is part of the navigation equipment. When a pilot pushes the HSI SEL button, it allows them to select the source of navigational data displayed on the Horizontal Situation Indicator, which can include information from navigation aids such as VOR (Very High Frequency Omnidirectional Range), GPS (Global Positioning System), or Localizer, among others. This is an essential function for pilots to maintain precise navigation and situational awareness during a flight.