The fixed length of the cycle a team plans, works and reviews in. Its value comes from repeating, not from being short.
Cadence is a property of how a team works, not of when anyone receives anything, which is why a team on a two week cadence can have an annual delivery cadence and both numbers be true. Almost all of the benefit comes from the interval repeating: a fixed rhythm lets testing, support and governance plan against a date they already know, and a predictable six week cycle is worth more to the organisation around it than a nominal two week one that keeps moving. Changing the interval rebases every measure denominated in it, so velocity across a cadence change is not a comparison.
Where this comes up
Predictive work gets an ending for free, the day the thing is handed over. Iterative work has to manufacture one, which is why so much of it never formally ends at all.
Cadence is the beat the work runs to. Delivery cadence is how often finished output reaches the people who asked for it. Teams conflate the two and then wonder why nobody feels the pace.
Iteration-based agile fixes the length of the cycle and commits a batch to it. Flow-based agile fixes the amount of work in progress and pulls the next item when a slot frees.