Every article on this site leans on the same handful of terms, and most of them are used more often than they are explained. This page is the explanation — one entry each, with the misunderstanding that usually travels with it.
Where Apple publishes no figure, this page says so. That is deliberate: a number invented for a glossary gets quoted as fact forever.
Battery terms
Battery cycle count — one cycle is 100% of capacity discharged in total, not one charge. Draining 50% and recharging, twice, is one cycle. You meet it when judging how worn a battery is. *Misunderstanding:* that charging often wears a battery out. Frequency of plugging in is irrelevant; total discharge is what counts. See Battery Health by Model.
Maximum capacity (battery health) — the percentage of original capacity the battery can still hold, shown at Settings → Battery → Battery Health & Charging. New is 100%; around 85% after two years is normal. *Misunderstanding:* that any drop is a defect. It is a consumable, and 80% is the published line where the system may limit peak performance — the reason old phones start feeling slow.
Optimized Battery Charging — iOS learns your routine and holds the charge at about 80% overnight, finishing to 100% just before you normally unplug. You meet it as "my phone stopped charging at 80%". *Misunderstanding:* that it is broken or that the battery is capping out. See Battery Saving.
Peak Performance Capability — a line on the same battery screen telling you whether iOS is limiting performance to cope with an aged battery. *Misunderstanding:* that a slow phone is always a software problem. Below 80% capacity the voltage cannot sustain full speed, so the system steps in.
Storage terms
System Data — a catch-all category in Settings → General → iPhone Storage for caches, logs, indexes, message attachments and temporary files the system manages itself. You meet it as the mysterious block that seems too large. *Misunderstanding:* that there is a button to delete it. There is not, and apps claiming to do so are guessing. Apple publishes no normal range for it — treat any site quoting one as estimating.
Optimize iPhone Storage (iCloud Photos) — keeps smaller versions on the phone and full-resolution originals in iCloud, fetching them on demand. You meet it when wondering whether iCloud photos are taking up space. *Misunderstanding:* that turning iCloud Photos off is harmless — the dialog offers "Download Photos and Videos" versus "Remove from iPhone", and the second can permanently lose items that had not finished uploading. See iCloud Photos and Storage.
Connection terms
eSIM — a SIM built into the phone rather than a plastic card, activated by downloading a carrier profile. You meet it when travelling or switching carriers. *Misunderstanding:* that it is only for new phones or that it works on a locked device — carrier lock still applies. A data-only travel eSIM cannot receive SMS, so anything using texted codes must stay on your main line. See Travel Settings.
5G bands (sub-6 and mmWave) — 5G runs on different frequency ranges: sub-6 GHz travels further and penetrates buildings; mmWave is much faster but short-range and easily blocked. You meet it when 5G is fast in one place and useless in another. *Misunderstanding:* that "5G" is one thing — it is several, with very different behaviour. See Weak Signal and Battery Drain.
APNs (Apple Push Notification service) — Apple's own channel for delivering notifications to your device. You meet it whenever notifications arrive (or do not). *Misunderstanding:* that Background App Refresh controls it. It does not — push and background refresh are two separate channels, which is why turning refresh on never fixes missing notifications. See Notifications Delayed or Missing.
Activation Lock — ties an iPhone to its owner's Apple Account so a stolen or erased device cannot be reactivated without those credentials. You meet it when selling or buying a used phone. *Misunderstanding:* that erasing is enough. You must sign out of the Apple Account first, or the buyer faces a locked device. See Moving to a New iPhone.
System terms
Thermal state — the level iOS reports to apps about how hot the device is: nominal, fair, serious, critical. Apps receive the state, never a temperature. You meet it as dimming, stutter or paused charging. *Misunderstanding:* that an app showing "CPU 47 °C" knows something iOS does not. It is estimating. See Thermal State and What Temperature Is Too Hot.
Ambient range (0 °C to 35 °C) — Apple's published operating range refers to the temperature of the surrounding air, not the phone's body. You meet it in every overheating discussion. *Misunderstanding:* that 35 °C is the phone's own limit. It is not — Apple publishes no body-temperature threshold at all, so judge heat by behaviour instead.
Background App Refresh — lets apps fetch new content while suspended. You meet it as a battery-saving setting people tell you to turn off. *Misunderstanding:* two of them — that it saves a lot of battery (it does not; the screen and the radio dominate), and that it is a permission governing notifications and location (it governs neither).