Beat
A beat is a repeating event or unit of time in music, which creates the rhythm that a person would tap or nod their head to while listening.
Each white tick represents a beat in the music
Beats usually have the same length as a quarter note (depending on the denominator of the time signature), which are contained between two adjacent white lines on the beatmap editor timeline. Equal amounts of beats are grouped into measures, which start from downbeats represented by large white ticks. The time between each beat depends on tempo.
Other notes are expressed through a beat by dividing or multiplying its length. The beatmap editor's beat snap divisor does that automatically for a mapper, allowing for changing the temporal granularity on the fly.
Since a beat snap divisor of 1/1 corresponds to a whole beat, one beat is also commonly referred to as a "1/1", a half of a beat (an eighth note) is called a "1/2", a quarter of a beat (a sixteenth note) is called a "1/4", etc.