Clare, The common meaning of "modal" for a one-line melody seems to be: something in the dorian, myxolydian, phrygian, locrian, or any other tonality other than aeolian and ionian. Of course the aeolian and ionian scales are scales like any other, so some word other than "modal" to refer to melodies-neither-aeolian-nor-ionian would be more logically consistent.

I am less certain of the meaning of "modal" when it is used to refer to harmonized music. My guess would be that it refers to techniques of harmonization which avoid the V-7 chord, or which avoid using in the harmony any note not in the scale of the melody. But that's just a guess.

T.