A peninsula — a piece of land surrounded by water on three sides (geographical term).
北海道の南には小さな半島がいくつかある。
There are several small peninsulas south of Hokkaido.
観光地としてその半島は人気がある。
That peninsula is popular as a tourist spot.
船から半島の先端がよく見えた。
You could clearly see the tip of the peninsula from the boat.
A colloquial and sensitive usage referring to Korea/the Korean peninsula; can be politically or culturally charged and should be used with caution.
その表現は一部で差別的に受け取られるため、公の場での使用は避けた方がいい。
That expression can be perceived as discriminatory by some, so it's better to avoid using it in public.
新聞では地理的な意味での『半島』が一般的に用いられている。
In newspapers, the geographical meaning of 'peninsula' is generally used.