What is Alternative Display Facility?

Legal Definition
Alternative display facility (ADF) is an equity trading facility created in the United States by the Financial Industry Regulatory Authority (FINRA), a self-regulatory organization (SRO). The ADF is an alternative to the exchange for publishing quotations and for comparing and reporting trades. This differs from a trading facility with execution capabilities (stock exchange) in that the exchange would simply send back to the owner of the displayed order a notice of execution. FINRA has operated an ADF since July 29, 2002.
-- Wikipedia