A financial market is complete if each and every agent, no matter his wealth and preferences, can reach his optimal consumption in each state of nature. Whether the market is complete or not has dramatic consequences on numerous issues such as valuation, hedging and replication, firms' investment and financing decisions and social welfare.
PONCET, P. (1999). Marchés financiers (complétude des ***°. In: Encyclopédie de la Gestion et du Management - E.G.M. 1st ed. Dalloz, pp. 772-775.