Generally a ‘charge’ is starting at 100% discharging the battery to zero or near zero, then charging to 100%. The battery will tolerate more shallow discharges, but it is difficult to provide exact life figures for shallow discharges. This battery technology likes to be kept charged. Sitting at 0% for extended periods will cause failure.

This is how Apple explains it (iPhone Battery and Performance - Apple Support and iPhone Battery and Performance - Apple Support):

“A normal battery is designed to retain up to 80% of its original capacity at 500 complete charge cycles when operating under normal conditions. The one-year warranty includes service coverage for a defective battery. If it is out of warranty, Apple offers battery service for a charge.”

“Charge your Apple lithium-ion battery whenever you want. There’s no need to let it discharge 100% before recharging. Apple lithium-ion batteries work in charge cycles. You complete one charge cycle when you’ve used (discharged) an amount that equals 100% of your battery’s capacity — but not necessarily all from one charge. For instance, you might use 75% of your battery’s capacity one day, then recharge it fully overnight. If you use 25% the next day, you will have discharged a total of 100%, and the two days will add up to one charge cycle. It could take several days to complete a cycle. The capacity of any type of battery will diminish after a certain amount of recharging. With lithium-ion batteries, the capacity diminishes slightly with each complete charge cycle.”