The term is typically used to specify a battery's expected life, as the number of charge cycles affects life more than the mere passage of time.
[1] Each battery is affected differently by charge cycles.
[2][3] In general, number of cycles for a rechargeable battery (the cycle life) indicates how many times it can undergo the process of complete charging and discharging until failure or starting to lose capacity.
[4][5][6][7] Apple Inc. clarifies that a charge cycle means using all the battery's capacity, but not necessarily by discharging it from 100% to 0%: "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.