def aa(i: Int) = i+2
object bb { def apply(i: Int) = i+2 }

scala> aa(1)
res7: Int = 3

scala> bb(1)
res8: Int = 3

scala> List(1,2,3).map(aa)
res5: List[Int] = List(3, 4, 5)

scala> List(1,2,3).map(bb)
<console>:7: error: type mismatch;
 found   : bb.type (with underlying type object bb)
 required: (Int) => ?
       List(1,2,3).map(bb)
                       ^