In Arvo Part's musical language, tintinnabulation refers to a technique he developed in the late 1970s, which (to a slightly fanciful imagination) recalls the ringing of small bells (in Latin, tintinnabuli). This technique predominates in his later work. A single chord (in fact, a triad) rings out again and again throughout the piece, while other voices (possibly even played on the same instruments) play a more melodic part. Tintinnabulation provides, at different junctures, a slow background rhythm, a surprising second voice, or even (in its absence), a suddenly accentuated silence.
A good piece to understand the power of tintinnabulation is Cantus in memoriam Benjamin Britten, although most any late Part composition will do.