Pandiatoncism is a term coined by Nicolas Slonimsky in the 1930's to describe a style of tonality that uses only the notes of a diatonic scale or mode, but in a way that strips away any sense of function.
Pandiatonic music tends to exhibit the following characteristics:
Only notes of a given scale or mode can be used.
Melodies tend to be angular.
Harmonies tend to avoid any sense of one note be more important than others.
Chords do not have doubled notes, as this may emphasize a note being more important than others.
Chords may be tertian (built on thrids) in nature, but are usually extended teritian (having ninths, elevenths, or thirteeths.
Simutaneities can be of any intervals. Chords built of seconds, fourths and fifths are acceptable.
In order to maintain interest, sudden key shifts are often used to keep the music from getting too static, so moving from a C major scale to an D-flat major scale suddenly would not necessarily be uncommon.
Some examples of Pandiatonicism to look up:
Ravel: Mother Goose Suite: The Magic Garden
Bartok: Little Pieces for Children, no. III
Debussy: La Cathedral Engloutie
Ravel: Rigaudon from Le tombeau de Couperin