All of us know intuitively what rhythm is. If you mention it in everyday conversation, everyone more or less knows what you’re talking about. But if you try to nail it down with a definition, it turns out to be quite slippery. Where exactly is it? Is it just the same thing as a beat? How can you describe rhythm in a way that is broad enough that it applies to many different kinds of rhythm but narrow enough that it distinguishes rhythm from other related concepts, like time, movement, or meter?
I have found that the most helpful resource for answering these questions is poetic theory. Poetry is an art that relies on rhythm yet is also the art that is most co-extensive with regular speech unlike, say, music which adds extra layers like melody, which can become confusing. While a lot of the points made here also apply to music, understanding rhythm primarily through poetry gives us the most straightforward way to understand rhythm in its own right and to show the connection between rhythm in art and everyday life.
While there certainly isn’t agreement within poetic theory on what rhythm is, there are agreed-upon components that must intersect in order for poetic rhythm to happen:
Layers of pattern
A metronome beat is not a rhythm by itself, it is only the bottom layer of a rhythm. A poem (or piece of music) layers larger chunks of symmetrical or asymmetrical repetitions over top of a beat. The top layer of a rhythm is the overall “narrative” arc that drives a piece towards the end. In poetry, this is usually associated with the meaning of the words. The relationship between this forward-moving arc and the very repetitive beat, which makes up the bottom layer of the rhythm, creates a tension that the poem mediates through middle layers of repetitions like lines, stanzas, phrases, etc. In other words, multiple layers of movement must come together in order for there to be rhythm.
In everyday life, of course, these patterns are not as intentionally and complexly orchestrated. However, the same a basic principle holds for ordinary thought and communication. While theoretically the beat of a metronome is not a rhythm by itself, in reality, whenever we hear a metronome beat our minds automatically begin to group the beats into larger scale patterns, thereby creating a rhythm. Likewise, English speech always involves more than one stress pattern: we use one stress-pattern to designate the overall meaning of a sentence (the words that we choose to emphasize), and this overlays the stress-pattern of the syllables of each word. If we zoom out even further, these patterns are also always interacting with the rhythms of heartbeat and breath, since these are the rhythms of the bodies that produce speech.
While layers of pattern are important, they are not enough on their own. It’s possible to pull out all of the layers that make up a rhythm and represent them in some way all together on paper (maybe like a conductor’s score), but that would not be the rhythm. It would only be a representation of rhythm. A real rhythm must also involve someone’s actual experience of that rhythm in time.
In poetry, rhythm is actually made by the human. The words or notes on the page are instructions for how to create the rhythm, they are not the rhythm itself. Reuven Tsur, in his research on the way cognition interacts with poetry, says that reading a poem is a process of inventing rhythmic solutions to new situations. A reader must try to hold together the musicality of the poem (for lack of a better term) with the meaning of the words. This is not always straightforward since the author will sometimes complicate the relationship between these two dimensions for certain effects. The reader must adapt the rhythm in response to unexpected poetic events. For example, when encountering a caesura – punctuation in the middle of a poetic line – the reader must find a way to both communicate the end of a sentence and retain the tempo and acoustic patterns of the poem as a whole. Rhythm is the solution to these poetic puzzles, but it is a solution that can only be performed by actually reading through the poem in real time.
Time is marked in rhythm through periodicity – the periodic recurrence of something, usually a strong beat (although there is a form of periodicity even in poetry which does not involve meter, such as through visual segmentation or syllable stress). The experience of periodicity is associated with the body. Language involves periodicity because we communicate through breath with its periodic alternation between inhalation and exhalation. In other words, rhythm is related to the way the body marks time. Poetry uses this same language, so the periodicity of the breath is also always built into poetry as the building blocks of its rhythm. The connection of rhythm to the body’s time is related to the purpose of rhythm. In poetry, rhythm functions to connect the meaning of a poem to the emotion of the reader or hearer. It reflects the levels of energy and the movements associated with various emotions, thereby evoking these emotions in the reader or hearer. This is just a more formal version of the way that rhythm functions in regular communication between people. We connect to others and experience empathy by adopting the rhythms of another through our own bodies (for more on this, see the post on why rhythm matters).
This is part of the reason that it is difficult to define rhythm. We can never really distance ourselves from it. We are always implicated in it. Nevertheless, based on these insights from poetic theory, my current working definition for rhythm is: the intersection of organized patterns through temporal experience. This definition is broad enough that it applies to many kinds of poetic and musical rhythm, as well as visual rhythms and rhythms in everyday events such as language and political and religious organization, but it’s more specific than just movement in that it’s the intersection of organized patterns. It’s more specific than just meter because it’s an intersection of organized patterns. And it’s more specific than just pattern in general because it must occur through temporal experience.
 See Richard D. Cureton, Rhythmic Phrasing in English Verse (London: Longman, 1992) for more detail on this dimension of rhythm.
 Series of stimuli are always rhythmically grouped in perception: Peter Auer, Frank Müller, and Elizabeth Couper-Kuhlen, Language in Time: The Rhythm and Tempo of Spoken Interaction (Oxford: Oxford University Press, 1999), 5.
 Denys Clement Wyatt Harding, Words into Rhythm: English Speech Rhythm in Verse and Prose (Cambridge: Cambridge University Press, 1976), 70-71, 154.
 Prose acts this out on a larger scale in which much of it may represent the continuous flow of everyday life and speech, punctuated by expressive eruptions. Harding, Words into Rhythm, 150.
 Reuven Tsur, Poetic Rhythm: Structure and Performance: An Empirical Study in Cognitive Poetics (Brighton: Sussex Academic, 2012).