Diatonic. I say it a lot, what does it mean?

This term comes up when we discuss scales and chord progressions and is a very important term to know when studying music theory. 

I'm leaving details about the origin and history of the word on this post as I often have some interesting linguistic discussions about it, more about this one than most of the other music terms we use.  

di·a·ton·ic

ˌdīəˈtänik/

adjective

Music

adjective: diatonic

1. (of a scale, interval, etc.) involving only notes proper to the prevailing key without chromatic alteration.

   • (of a melody or harmony) constructed from a diatonic scale.

      

Origin

early 17th century (denoting a tetrachord divided into two tones and a lower semitone, or ancient Greek music based on this): from French diatonique, or via late Latin from Greek diatonikos ‘at intervals of a tone,’ from dia ‘through’ + tonos ‘tone.’