a large mass of land projecting into a body of water
"The Florida peninsula is a vast stretch of land that juts out into the Atlantic Ocean and the Gulf of Mexico."
A piece of land projecting into water from a larger land mass.
"The long peninsula juts out into the ocean, separated from the mainland by narrow straits on either side."
In plain English: A peninsula is a piece of land that sticks out into water and connects to larger land on only one side.
"Florida is a large peninsula surrounded by water on three sides."
Usage: A peninsula is defined by its connection to the mainland, distinguishing it from an island which is completely surrounded by water. Use this term when describing any large body of land that juts out into an ocean or lake while remaining attached at one end.
The word peninsula comes from the Latin paeninsula, which combines paene (meaning "almost") and insula ("island"). It entered English to describe a piece of land that is surrounded by water on all sides except for one.