Port is the seafaring term for the left side of the boat or ship, from the point of view of a person aboard the ship, facing the front, or bow. This term is also used for spacecraft and airplanes. The port side of a ship does not change depending on which way a person is facing. The opposite is starboard.
The original English nautical term for the left side of a ship was larboard, which came from the Middle English term for "the loading side." As the word rhymes with "starboard," it was easily confused, especially during high winds or in loud environments, and this could cause significant problems when attempting to follow shouted commands. In the 1500s, the term was changed to "port," likely since that was the side of the ship where dockworkers normally load cargo from the port.
An alternate theory for the origin of the word is that the English word came either from the Latin word porta, which means "door" or "gate" or portus, meaning "harbor." This could have again referred to the side of the ship where the dockworkers loaded cargo or to the gate or door opened to load the cargo.
Whalers continued to use the term "larboard" up until the 1850s, long after English-speaking merchant mariners adopted the other term. Captain Robert FitzRoy of Charles Darwin's ship, the HMS Beagle, instructed his crew to substitute the term "larboard" with "port" in the late 1820s. This may have prompted England's Royal Navy to adopt the term in 1844.
Seagoing vessels and airplanes all have a red light on the port side, a green light on the starboard side, and a white light on the aft, or rear. This can help people remember which side of the ship is which with a few simply associations. The port side of a ship has a red light, and the wine with the same name is generally red. "Port" and "left" also both have four letters.