What does the linux ‘top’ CPU fields stand for?

The first few lines of the top command looks similar to this:

Tasks: 137 total, 1 running, 136 sleeping, 0 stopped, 0 zombie
Cpu(s): 0.0%us, 4.2%sy, 90.0%ni, 0.0%id, 0.0%wa, 0.0%hi, 0.0%si, 5.8%st
Mem: 786432k total, 715152k used, 71280k free, 61012k buffers
Swap: 2096472k total, 18304k used, 2078168k free, 168068k cached

The fields on the second line breaks down the CPU usage into 8 categories.

us: user CPU used by user processes
sy: system CPU used by system/kernel processes
ni: nice CPU used by processes that were reniced
id: idle CPU not used
wa: io wait Essentially idle CPU waiting on IO devices
hi: hardware irq CPU used to service hardware IRQs
si: software irq CPU used to service soft IRQs
st: steal time CPU time which the hypervisor dedicated (or ‘stole’) for other guests in the system.

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