iStat terminology

CPU

User — Percentage of CPU time used by the current user. 100% represents one core being used fully, so you may see values of 150% or higher if you have several CPU cores.

System — Percentage of CPU time used by tasks that belong to the system (eg. processes owned by root, windowserver etc).

Nice — Percentage of CPU time used by tasks that are running using nice. These processes are using a non standard priority level to give them more or less priority.

Idle — Percentage of time that the CPU or CPUs were idle.

Apple provide further CPU information in their Use Activity Monitor on your Mac help document.

Memory

Wired memory — This is memory that applications or the system needs immediate access to, so it can’t be cached to disk. It will vary depending on what applications you’re using.

Active memory — This is memory that is actively being used.

Inactive memory — This memory is no longer being used and has been cached to disk. It’ll remain in RAM until another application needs the space.

Free memory — This memory is not being used.

App memory — Memory used by apps and their processes.

Compressed memory — Memory that is compressed to make more space available to other processes. Memory is compressed to limit swapping to disk.

Apple provide further memory information in their Use Activity Monitor on your Mac help document.

Battery

Battery Health — This is a comparison between your current battery capacity and capacity of it when it was new. For example, if your battery lasted 5 hours when it was new, a figure of 50% suggests your battery should last roughly 2.5 hours now.

Cycles — The amount of battery cycles the battery has had. One cycle is defined as a complete discharge and charge of the battery, but partial discharge and charges also count. So, two half discharge and charges are the same as one complete discharge.

For some tips on how to get the most out of your laptop’s battery, please visit this Apple support page.