The word Dia-tonic really comes from a couple explanations, but Dia doesn't mean a two note scale like Penta means 5 and octa mean 8.

One explanation is, it's a scale that consists of all whole steps, except for TWO half steps. This is consistant for all the modes of the Diatonic scale.

The other explanation is: It is a scale with TWO Roots. The 1 equaling the Root of the Major scale, and the 6 equaling the Root of the Natural Minor scale.

Just some tid bits :)