Document filed with the government with a mental view or survey of a venture.  It outlines the proposed investment plan to give enough information to prospective investors to make educated decisions whether to take a position in the security or not.  A legal document describing something that is being offered for sale, with all disclosures required by applicable state and federal laws. 
From prospectus "view, outlook," extensive view of the landscape is from 1530s. "Expectations" is from 1660s. Spot giving prospects of ore" from 1839. Prospector in this sense is from 1857.