In Scala, what exactly does 'val a: A = _' (underscore) mean?

What exactly does val a: A = _ initialize a value to? Is this a typed null? Thanks.

val a: A = _ is a compile error. For example:

scala> val a: String = _
<console>:1: error: unbound placeholder parameter
       val a: String = _
                       ^

What does work is var a: A = _ (note var instead of val). As Chuck says in his answer, this initialises the variable to a default value based on its type.

From the Scala Language Specification:

A variable definition var x: T = _ can appear only as a member of a template. It introduces a mutable field with type T and a default initial value. The default value depends on the type T as follows:

default	type T
0	Int or one of its subrange types
0L	Long
0.0f	Float
0.0d	Double
false	Boolean
()	Unit
null	all other types