What is Schedule 13D?

Legal Definition
Schedule 13D is an SEC filing that must be submitted to the US Securities and Exchange Commission within 10 days, by anyone who acquires beneficial ownership of more than 5% of any class of publicly traded securities in a public company. A filer must promptly update its Schedule 13D filing to reflect any material change in the facts disclosed, including, among other things, the acquisition or disposition of 1% or more of the class of securities that are the subject of the filing.
-- Wikipedia
Legal Definition
A filing required by the Securities and Exchange Commission where ownership is acquired of more than 5% of the stock in company trading publicly.