The regular or irregular vocal rhythm used by the quarterback when calling out plays at the line of scrimmage.
Cadence, by its normal definition, refers to the vocal rhythm of a speaker or singer and it is considered to be no different than the cadence used by a quarterback in a football game. Quarterbacks call out plays or audibles at the line of scrimmage and use a particular cadence or tempo prior to the snap that all the players on the offensive side of the ball are familiar with. Regardless of the actual words used, offensive players all practice and learn the differing cadences used by their quarterback.
Cadences can be rhythmic, following a specific tempo or non-rhythmic, in which the quarterback staggers his speech. Although rhythmic cadences are the easiest to practice and use, a quarterback who gets into a rhythmic pattern becomes predictable, allowing defenses to time his snap count and get off the line more quickly than the offense. Non-rhythmic cadences are therefore used frequently by quarterbacks in an attempt to draw defenders offside, staggering their speech to fake an earlier ball snap.