What count as a cycle (battery life)

[gauvins]

Expected life of a battery is typically expressed in cycles. Say 200 for lead, 600 for AGM, 1000 for lithium.

What counts as a cycle?

If batteries are always kept between, say, 70% and 85%, i.e. a mini cycle of 15%, would 6 such cycles count as a full cycle? Or what?

If a battery is kept fully charged during the off season (10 months a year in our case), does it mean no cycling no ageing at all?

All I could find were examples of cycles going from full charge to the suggested minimum for a given chemistry. Say 50% for deep discharge, 20% for AGM, 0% for lithium. (I know that one should avoid getting even close to these values in order to optimize battery lives, but I have no idea about the relationship between usual depth of discharge and life expectancy (expressed in total Ah stored and pulled from a battery)