Why a tunnel diode (TD) is called a diode? Traditionally, a diode is a 2-terminal device that limits current to flow in one direction, i.e. a rectifier. But a "tunnel diode", according to wikipedia, is not rectifying:

In the tunnel diode, the dopant concentrations in the p and n layers are increased to the point where the reverse breakdown voltage becomes zero and the diode conducts in the reverse direction. 

Then why call this device a diode if it's not rectifying? Isn't it "just" another device that exhibits negative differential resistance?
 A: It's called a diode because the device has two terminals.  Devices that have three terminals are called triodes, and those with five pentodes.   Words of that type have fallen by the wayside except in the realm of vacuum tube electronics ... except for the word diode, which has hung on.
Note that the Wikipedia article that you cite refers specifically to the tunnel diode, which is a type of semiconductor diode.  If you go to the Wikipedia page for diode you will find this:

In electronics, a diode is a two-terminal electronic component ...

The vacuum tube diode was, of course, around long before the semiconductor diode.  The vacuum tube diode has asymmetric conductance.  Both types of diode play similar roles in electronic circuits.  When the semiconductor device was invented, the natural name for is was "semiconductor diode".  Since we don't see many vacuum tube diodes anymore, we shorten the name to diode, and there is little chance for confusion.
Update following question edit
It seems that the first semiconductor diode was named in analogy to the vacuum tube diode.  Subsequently, several related devices, all employing semiconductor junctions, and all having asymmetric conductance, were developed.  I suppose the word "diode" became the name of the category of such devices.  (I think negative differential resistance has nothing to do with the name.)
A: The negative differential resistance creates a special phenomena: under a certain bias and certain incident electron energies, the transmission function through the double barrier is nearly zero. In other words, for a finite voltage domain, the current is nearly zero, i.e. a rectifying behaviour. This can be referred to as "generalized diode", since the rectifying behaviour is not passing through the origin of the IV curve.
