This is a Merriam Webster def of the word cycle:

1: an interval of time during which a sequence of a recurring succession of events or phenomena is completed

This is a Merriam Webster def of the word interval

1a: a space of time between events or states

At first, I thought that "interval of time" simply means "a period of time" but after referring to a dictionary I realized that there isn't a definition like that for this word. As it relates to the first definition of the word "cycle", does this mean that the meaning of that definition is "the period between each cycle that takes place"? It's weird though because things that are happening during the duration of the cycle are also the cycle, but the definition seems to me like it only talks about the time between. Or maybe interval simply means period and I missed something.

  • 2
    Why the downvoting? The question isn't a simple request for definition. And the first MW definition is actually a tad confusing, even to my (native speaker) eyes. I'm not at all saying the downvoters are wrong, but it would be useful to know their reasons, at very least so we can all improve our question-writing ;-)
    – tkp
    Mar 29 at 20:11
  • I don't like the M-W definition (words I never thought I hear myself say). I'd say a cycle is a span of time.... An "interval" is the time between events, not the time when the event happens.
    – gotube
    Apr 22 at 0:38
  • Did you check other dictionaries? Cambridge has a clear definition.
    – gotube
    Apr 22 at 0:49

2 Answers 2


So, there are three key words at play in the Merriam Webster definitions: cycle, interval, and space. And from my point of view, the definitions are fairly clear. However, in your commentary, you introduce a fourth word: period. That doesn't necessarily confuse things, but it might do. The reason is that the word "period" has a technical definition in Physics. So, avoiding that word for now, I interpret the essence of the MW definitions and words as follows:

A space of time is the simplest. It's just some time; a chunk of time, if you like; a blob; a clump; and so on. The only additional aspect of the concept of "space" is that we are talking about contiguous time; i.e. a single chunk/blob/clump.

An interval of time is just a space with some kind of significance associated with the start and end points. Obviously all (finite) spaces of time have a start and end point, but interval is used when those points are of some relevance in the context.


A cycle is an interval that is subject to being repeated in a way that is in some way, more or less, predictable, analyzable, significant, and so on.

So, a cycle is a special form of interval, which in turn is a special form of space (of time).

Finally, back to period. In Physics, the period of a wave (more generally, of harmonic motion) is the reciprocal (i.e. 1 over) the frequency. In that sense, you are correct that "interval" is synonymous with "period". And in everyday use, they are probably safely interchangeable.

  • "Interval" also has a technical definition in physics, it is the distance between two space time events, or the distance between two points in a multi-dimensional manifold. Each is a generalization of the 3D-definition of distance, the square root of a sum of squares, one along each dimension Neither the technical sense of "interval" nor that of "period." is very relevant to the definition of "cycle" in the question. Mar 30 at 0:41
  • You're probably right. I was guessing that maybe the reason Merriam Webster used the rather clumsy "space of time" instead of "period of time" was to avoid confusion via the Physics meaning of the latter. Shrug.
    – tkp
    Mar 30 at 2:37

I've already given one answer, but I wanted to give another based on one thing you say which you suggest may be at the heart of what's confusing you. First, let me set up an example which may help clarify:

Consider someone working out at a gym. Their workout is based on what in gym terminology we might call a "circuit". It might look like this:

  1. one set of five bench presses
  2. rest for 30 seconds
  3. one set of five squats
  4. rest for 30 seconds
  5. one set of five deadlifts

And then their gym session as a whole might consist of three of those circuits, with 90 seconds rest between each.

Now referring back to the Merriam Webster definitions, it might be tempting to say that "cycle" is simply a synonym for "circuit" in the above. But in fact the MW definition is such that "cycle" refers very specifically and only to the time itself, and not to the various exercises.

So the thing you say that may be relevant is:

It's weird though because things that are happening during the duration of the cycle are also the cycle, but the definition seems to me like it only talks about the time between.

But that's not exactly correct. It is not the case that "things that are happening during the duration of the cycle [i.e. the five items in the circuit] are also the cycle" Applied to the gym example, the word "cycle" means nothing more than the blob/lump/space of time from the start of the first bench press in a circuit, to the last deadlift in that same circuit.

Also, it's not clear, in the above quote from your question, what you mean by "the time between" Do you mean the time between exercise within a circuit -- i.e. 30 seconds? Or do you mean the time between circuits -- i.e. 90 seconds? (Or maybe even the time between each benchpress, each squat, etc -- i.e. not specified, but usually very little if any time!)

In summary, using my example:

Interval could correctly be used to refer to any of the times I mention: the 30 seconds between exercises, or the 90 seconds between circuits. The key is that both have meaningful start and end points

Cycle could correctly be used to refer to any one sequence of the five items in my list. The key is that the entire sequence of five is subject to repetition.

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