Afternoon is the time of day from 12:00 (noon) to -depending upon context- evening, sunset, or 18:00. Its use is often quite subjective.
This should not be confused with after noon (two words) a translation of post meridiem (p.m.) as used in the 12-hour clock
a time of day from noon to midnight.
In Australia and New Zealand, the word "arvo" has the same meaning as afternoon.