Glossary
Compressed memory
Memory that macOS has compressed in RAM to make room, instead of writing it to disk.
What it is
When memory gets tight, macOS compresses pages that haven't been used recently. Compressed pages take less space and stay in RAM, which is much faster than writing them to swap on the SSD.
Some compression is normal on any busy Mac. A large, growing compressed figure means macOS is working harder to fit everything.
Why it matters
Compression is the step before swapping. A compressed figure that keeps climbing, together with rising memory pressure, is an early sign that something is growing.
In Activity Monitor
“Compressed” under the Memory Pressure graph, and a per-process Compressed Memory column you can add with View → Columns.
In iKnowMyMac
The Memory tab breakdown and the memory-by-type donut in the Overview.
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