On November 17, 1970, U.S. Patent No. 3541541 issued to Douglas Engelbart on an X-Y Position Indicator for a Display System (computer mouse):

Engelbart he first recorded his thoughts about something he initially called a “bug” in his personal notebook on November 14, 1963. He wrote that the “bug” would be “easier” and “more natural” to use. The earliest known written use of the term “mouse” in reference to a computer X-Y pointing device is in a July 1965 publication, “Computer-Aided Display Control”. The name is widely believed to be form the resemblance of the shape and size of the device resembling that of a mouse, with the cord resembling its tail. Of course, today’s wireless mice eliminate some of the resemblance.