The Check Clearing for the 21st Century Act (or Check 21 Act) is a United States federal law, Pub.L. 108-100, enacted into law October 28, 2003 by the 108th Congress. It took effect one year later, on October 28, 2004.
The law allows the recipient of the original paper check to create a digital version of the original check””called a “substitute check,” thereby eliminating the need for further handling of the physical document. An implication of the law is that it is legal for anyone to use a computer scanner to capture images of checks and deposit them electronically, a process known as remote deposit.
(Definition courtesy of Wikipedia)