A diatonic scale is defined as an ordered collection of pitches. However, given the many modes of the diatonic scales, what is meant when someone says "the" diatonic scale? Wikipedia says it's an ordered set of pitch classes, even though it says that a scale is an ordered set of pitches, so I wasn't sure if one can reasonably call a collection of pitch classes a scale.
Most people will use them interchangeably and you will be understood either way, but there is a reason for the distinction.
The diatonic scale refers to the pattern which is utilized inside the diationc scale itself which consists of the Major (Ionian), Dorian, Phryigian, Lydian, Mixolydian, Minor (Aeolian), or Locrian. Think of the major scale pattern, WWHWWWH, where you can start on any degree and the pattern will be the same, just your starting in a different degree.
A diatonic scale specifically refers to only one of the subsets.
A scale is diatonic if it has seven scale degrees separated by the either the following pattern or any of its rotations (W= whole step, H= half step):
Any scale starting on any pitch that has this pattern of whole steps and half steps is a diatonic even if the pattern starts somewhere other than the beginning. There is no need to distinguish between a diatonic scale and the diatonic scale because diatonic means both that there are seven notes in the scale, and that they are in a given intervallic disposition. To contrast this, any scale with just five notes is technically "pentatonic" but when most people refer to it, they mean the scale that has this interval pattern (W= whole step, m3= minor 3rd):
Here it is important to distinguish between scales with this particular pattern and other ones that have five notes, but follow a different intervallic pattern such as the "Japanese Mode". Note that the linked article refers to this as a pentatonic scale. If you want to specify that you're referring to the scale with the W-W-m3-W-m3 pattern, you can say it's the pentatonic scale. Note that this distinction is not necessary with the diatonic scale, because the term diatonic refers to both the number of notes, 7, and the interallic composition of the scale unlike pentatonic, for example, that only technically refers to the number of notes.
A diatonic scale is a scale made of seven notes, with five whole tones and two half tones. Therefore a major scale is a diatonic scale, but a diatonic scale might not be a major scale.
Some may use "the diatonic scale" to name the "major scale" interchangeably, but that is in theory wrong. In some contexts you might be understood if you use "diatonic" to designate the "major scale", I don't know; but I can tell you in the environment of classical music, you don't use these terms interchangeably, because they mean different things.
"The" diatonic scale likely means the collection of "white keys" on a keyboard without indicating a particular of the church modes. So it's the notes of C major but without a particular tonic in mind.
Possibly even used for any scale with the same sequence of whole- and half-tone steps, distinguishing it from chromatic scales and possibly "unnatural" scales like melodic/harmonic/gypsy minor.
The simplest definitions for diatonic and chromatic are:
Diatonic is all the notes "inside" the scale (whatever that scale might be).
Chromatic refers to notes that are "outside" that scale.
Thus, the seven diatonic chords of the major scale are like a family who all share the same parent scale.
Furthermore, diatonic chords can be created within any scale. E.g. a C-wholetone scale contains a family of 6 diatonic chords - each of them an augmented chord, diatonically.
These are deliberately simplistic definitions that I use to explain these concepts to college students who take my Beginner Keyboard course as an elective. (i.e. adult beginners with no musical experience). Student feedback suggests they are effective, easily understood definitions that are useful for practical musical purposes.