An intercom is a communication system that lets people talk to each other between different rooms or areas of a building without leaving where they are. Schools use intercoms when the principal's voice suddenly comes through a speaker in your classroom to make an announcement. Apartment buildings have intercoms at their front doors so visitors can buzz up to tell residents they've arrived.

When your teacher presses a button to reply to the office over the classroom intercom, she's using a two-way system. Some intercoms only work one direction, like when morning announcements go out to every classroom but students can't talk back through them.

Homes sometimes have intercoms too. Before everyone had cell phones, families in large houses used them to call each other to dinner or ask questions without shouting up the stairs. Hospitals use intercoms so nurses can communicate quickly between stations. Even airplanes have them, allowing pilots to speak to flight attendants or make those welcome-aboard announcements passengers hear through overhead speakers.

Modern intercoms might use wireless technology or connect through a building's internet, but the basic purpose remains the same: letting people communicate across distances within a building or complex without walking to find each other.