an afternoon is a period of time in the day between noon and evening, ideally between the hours of 2 and 5 pm. As such, noun usage of the word is used to refer to a certain time in the day, or the hours comprising it.
1. I usually take a nap in the afternoon.
2. We are planning to have a picnic in the afternoon.
Generally, when referring to a specific afternoon, the definite article is used ('the afternoon'). When referring to a general one, no article is used ('afternoon sun'). Common mistakes include using the word to mean evening ('In the afternoon, I went to dinner').
when used as an adjective, afternoon is used to modify a noun to refer to various aspects of the afternoon, or to describe a quality that is typical of that time of day.
1. We experienced an afternoon chill outside.
2. I had an afternoon migraine, so had to take an early nap.
3. She took an afternoon stroll in the park.
This form of the word does not change regardless of the noun it is modifying; it always retains the same spelling and pronunciation. When referring to an afternoon in a specific day, the definite article is used ('the afternoon chill').