An investment company that buys and holds a fixed number of shares until the trust's termination date. this is the structure used by some ETFs. A portfolio may contain one of several different types of securities.
UITs are regulated maily under the Investment Company Act of 1940 and the rules adopted under that Act., in particular Section 4 and Section 26.
Section 4 reports:
"Unit investment trust" means an investment company which
- is organized under a trust indenture, contract of custodianship or agency, or similar instrument,
- does not have a board of directors, and
- issues only redeemable securities, each of which represents an undivided interest in a unit of specified securities; but does not include a voting trust.
To know more about UITs you can read:
http://www.sec.gov/answers/uit.htm