Definition
Varying rep range and load within the week (daily undulating) or from week to week, rather than holding one scheme for a whole block.
A daily undulating week for one lift: Monday 4 x 8 at 70%, Wednesday 5 x 3 at 85%, Friday 3 x 12 at 60%. Strength and size get trained together, boredom drops, and intermediates who train a lift two or three times a week tend to respond well.
Weekly undulating does the same across weeks: a volume week, a heavy week, a moderate week, repeat.
The cost is tracking. Progress has to be compared like with like, heavy day against heavy day, or the log looks like noise. It also needs a lift trained at least twice a week to make sense.