Afternoon is the period between noon and evening. The term lacks a precise definition because it was in use long before accurate time-pieces became widespread. Hence, words and phrases that refer to general periods in the day were more practical.
The term should not be confused with "after noon" (two separate words), which is a translation of the Latin post meridiem (p.m.), meaning a time of day between 12:00 midday and 12:00 midnight.
In Australia and New Zealand, "arvo" is slang for afternoon.