This is the 290th most frequent French word.
seul
Alone; only.
In this sentence, "seul" describes the subject's desire to be in solitude.
Il veut rester seul ce soir.
He wants to stay alone tonight.
Here, "seule" characterizes the island as being solitary in its location.
Cette île est une île seule au milieu de l'océan.
This island is a lone island in the middle of the ocean.
In this context, "seule" is used to indicate exclusivity, meaning "only one."