Nice value
A process's politeness number — how willing it is to give up CPU time. It runs from −20 to 19.
The name is literal: a high value means the process is nice to others and gets less time. The default is 0. Only root can set negative values, that is, make a process more demanding. An ordinary user can only be nicer, never greedier, and cannot undo a raise either.
The scale is not linear — each step is roughly a ten per cent difference in allocated share, so the distance from −20 to 19 is substantial. Typical use is running something heavy but unimportant, such as a backup or a transcode, with nice -n 19 so it only takes time nobody else wants. Note that nice affects CPU only; for disk there is ionice separately.