pro·lu·sion 
(prō-lzhən)
(prō-lzhən)n.
1. A preliminary exercise.
2. An essay written as a preface to a more detailed work.
[Latin prōlūsiō, prōlūsiōn-, from prōlūsus, past participle of prōlūdere, to practice beforehand : prō-, before; see PRO-1 + lūdere, to play; see leid- in the Appendix of Indo-European roots.]
pro·luso·ry (-sə-rē, -zə-) adj.