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The niceness of a process decreases with increasing process priority.
Extract from Beginning Linux Programming 4th Edition, Pg 169 :
The default priority is 0. Positive priorities are used for background tasks that run when no other higher priority task is ready to run. Negative priorities cause a program to run more frequently, taking a larger share of the available CPU time. The range of valid priorities is -20 to +20. This is often confusing because the higher the numerical value, the lower the execution precedence.
Is there any special reason for negative values corresponding to higher process priority (as opposed to increasing priority for higher niceness valued processes) ?
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The nice value determines the priority of the process. The higher the value, the lower the priority--the "nicer" the process is to other processes. The default nice value is 0 on Linux workstations.
By making the niceness value higher you will ensure that critical system processes will get higher CPU priority than the jobs ran by the users.
nice is used to invoke a utility or shell script with a particular CPU priority, thus giving the process more or less CPU time than other processes. A niceness of -20 is the highest priority and 19 is the lowest priority. The default niceness for processes is inherited from its parent process and is usually 0.
A process' priority can range between 0 (lowest priority) and 127 (highest priority). User mode processes run at lower priorities (lower values) than system mode processes. A user mode process can have a priority of 0 to 65, whereas a system mode process has a priority of 66 to 95.
@Ewald's answer is correct, as is confirmed by Jerry Peek et al. in Unix Power Tools (O'Reilly, 2007, p. 507):
This is why the nice number is usually called niceness: a job with a high niceness is very kind to the users of your system (i.e., it runs at low priority), while a job with little niceness hogs the CPU. The term "niceness" is awkward, like the priority system itself. Unfortunately, it's the only term that is both accurate (nice numbers are used to compute the priorities but are not the priorities themselves) and avoids horrible circumlocutions ("increasing the priority means lowering the priority...").
Nice has had this meaning since at least V6 Unix, but the V6 manual never explains this explicitly. The range of allowed values was -220 through +20, with negative numbers reserved for the superuser. The range was changed to -20 through +20 in V7.
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