What exactly is one "cycle" on a battery ?

[Kithmo]

It must be an age thing but I'm a bit confused as to what exactly a cycle is on a battery.
Is it one charge from 0-100% ?
Is it each charge from whatever to whatever % ?
Is it a collective number of charges that add up to 100% ?
For example, if I charge my battery say 20 times from 50-80%, is that 20 cycles or is it 20 x 30% = 600%, 6 cycles ?

 

[Deleted member 3150]

Oh if it were only that simple!

Peukert’s Law so much fun!​

BU-808: How to Prolong Lithium-based Batteries

Manufacturers take a conservative approach and specify the life of Li-ion in most consumer products as being between 300 and 500 discharge/charge cycles.…

batteryuniversity.com

and
What Is A Battery Life Cycle? | Dragonfly Energy

Best way to look at it is manufactures benchmark a test on a cycle being one complete discharge - recharge event. Reality is that it never happens that way in practical usage. So maybe forget about that and work on life cycle averages and warranty guarantees on degradation over years of ownership.