I think it is each segment of a line of music that is separated by a vertical line.
meter
Although music has had rhythm since music was first conceived, the person who is generally held responsible for the invention of measured, fixed rhythm is Franco of Cologne.
units of time.
units of time
Music can be measured in terms of tempo, which is the speed of the music, rhythm, which is the pattern of beats, and structure, which is the organization of the music into sections like verses and choruses. Tempo is measured in beats per minute, rhythm is measured by the arrangement of notes and rests, and structure is determined by the order and repetition of musical elements.
A rhythm unit refers to a specific segment of time that constitutes a regular pattern in music. It is the basic building block of musical rhythm, often measured in beats or pulses within a given time signature. Rhythm units help to establish the tempo and structure of a musical composition.
" .... Harmonic rhythm is how often chords change in a passage of music, and is measured in note values. For example, if chords change every half-note, we say "the harmonic rhythm is a half-note." Or, if the harmony changes every quarter note, we say "the harmonic rhythm is a quarter-note." Sometimes the harmonic rhythm is a combination of note values that change at a regular rhythm, such as a half-note followed by a quarter-note ......" from >http://music-theory.ascensionsounds.com/tag/harmonic-rhythm/
The rhythm of early notated music came from a few different sources. One idea is that it was first notated with the Greeks.
Measured rhythm in music composition is used to establish a consistent beat and tempo, providing a framework for the arrangement of musical elements. By adhering to a specific rhythm, composers can create a sense of structure and coherence in their music, guiding the listener through the piece and helping to unify different sections. This rhythmic consistency helps to organize the musical material and create a cohesive and engaging listening experience.
The measured arrangement of words in poetry is called "meter." Meter involves organizing and counting stressed and unstressed syllables in a line of poetry to create rhythm and structure.
rhythm
external rhythm and internal rhythm