Diminution can take different meanings in different contexts. In the context of the Busse Berger article, diminution of a durational pattern in a piece of music is its recurrence or simultaneous performance by another part in shorter rhythms (lesser duration), usually in notes of half the original length.
It can also refer to scales, chords, or intervals, in which case there is a specific system for determining the intervals between notes.
Diminution is also a term applied to the subdivision of a given time-span that is represented by a given tone into two or more tones. As such, it is one of the primary means of melodic elaboration or variation.
(Added by Ève Poudrier, 2/16/10)