osamljen

1. (of a person) lonely; alone
   Posle penzije se osećao osamljeno. — After retiring he felt lonely.
2. (of a place or thing) isolated; solitary
   Na brdu je stajala osamljena kuća. — A solitary house stood on the hill.
   To je bio osamljen slučaj. — That was an isolated case.

For a lonely person, usamljen is more common.