Marking the close is the practice of buying a security at the very end of the trading day at a significantly higher price than the current price of the security. The purpose of the practice of marking the close is to raise the closing price of the security, making it appear to be higher-valued than it actually is.
The following is an example of a case law referring to marking the close:
Marking the close is the practice of placing late-day orders to raise the reported closing price of the stock. [SEC v. Competitive Techs., Inc., 2005 U.S. Dist. LEXIS 43349 (D. Conn. July 21, 2005)].