The term I (SPEED OF SOUND) is represented by the crossword solution MACH, which is an acronym for 'Mach Number'. Mach Number is a measure of the SPEED of an object relative to the SPEED of SOUND in the surrounding medium. It is named after Austrian physicist Ernst Mach, who first proposed the concept in the late 19th century. The Mach Number is typically expressed as a decimal fraction, with 1 being the SPEED of SOUND in the medium.