A type of ‘stand-by’ mode employed in certain types of equipment. Sleep specifically has been associated with the Macintosh computer over the years. In that case it turns off the video output of the machine, spins down or turns off the hard drive, and disables aspects of a number of other functions (networking, etc.), but leaves power to RAM so any running applications don’t have to be restarted when the computer is revived. The overall purpose is so it can be returned to the state it was prior to being put to sleep much faster than rebooting and relaunching all of the programs. The specific characteristics of sleep mode will differ with other equipment, but in general you can always think of it as a stand-by mode.