Preludes usually serve as introductory movements in multi-movement works, though they also may stand alone. The formal structure varies widely. The title may also refer simply to the piece of music played at the beginning of a worship service.
Example videos:
Philippe Lefebvre – Prélude – 01.05.2011 – Notre Dame
Loïc Mallié – Entrée improvisée – La Trinité, Paris
Olivier Penin – Prelude – St. Clotilde
Pierre Pincemaille – Prélude – St. Denis
William Porter – O dass ich tausend Zunge hätte – Prelude
Johann Vexo – Messe du dimanche soir 07.04.2013 – Prélude
Johann Vexo – Vêpres 07.04.2013 – Prélude