Final answer:
The first beat of most measures in music is referred to as the downbeat, which is typically the strongest and most accentuated beat in a measure.
Step-by-step explanation:
The first beat of most measures in music is called the downbeat. The downbeat is typically the strongest beat in a measure where the conductor's baton moves downward. It is the beat that musicians often use to orient themselves within the rhythm of the piece and is usually stressed when counting the beats in a measure.
In contrast to the downbeat, the other terms refer to different aspects of the rhythm: an upbeat is often a weak beat that leads into the downbeat, a backbeat typically refers to beats 2 and 4 in 4/4 time, and an offbeat is any beat that is not the main beat (for example, in between the main beats of a measure).