A battery cycle is one full discharge equivalent (e.g., 50% drain twice = 1 cycle). Lithium-ion batteries typically last 500–1000 cycles before capacity drops below 80%.
Apple's typical spec: maintains 80% capacity at 1000 cycles for iPhone 15+, 500 cycles on earlier models.
Factors that accelerate degradation: Full 0–100% charges (vs partial 20–80%). High temperatures (charging in hot car). Fast charging at high wattage continuously.
To extend battery life: use 20–80% charge range when possible, avoid heat exposure, use slower charging overnight. Most modern phones offer "Optimized Charging" that learns your schedule and pauses at 80% until just before you wake.