A peninsula is a piece of land extending out into a body of water that is still connected to mainland or a piece of land that is bordered by water on three sides but connected to mainland, for example, the upper and lower peninsulas of the state of Michigan, the peninsula of downtown Vancouver, or the Niagara peninsula.
Mainland is a contiguous landmass that is larger and often politically, economically and/or demographically more significant than politically associated remote territories, such as exclaves or oceanic islands situated outside the continental shelf.
MAPA 2: PENÍNSULAS by Francisco Quintana Toret
The surrounding water is usually understood to be continuous, though not necessarily named as a single body of water.
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Peninsulas are not always named as such; one can also be a headland, cape, island promontory, bill, point, or spit.
A promontory is a raised mass of land that projects into a lowland or lower body of water.
A point is generally considered a tapering piece of land projecting into a body of water that is less prominent than a cape.
A river which courses through a very tight meander is also sometimes said to form a "peninsula" within the loop of water.
In English, the plural of peninsula is peninsulas or, less commonly, peninsulae.
The plural, in many languages, is one of the values of the grammatical category of number.