Afternoon is often defined as the time period between noon and evening. If this definition is adopted, the specific range of time varies in one direction: noon is at 12:00 (for simplicity, using the 24 clock ("military time")), but the boundary between afternoon and evening has no standard definition. The time period of evening is subjective, but it is often socially considered to start around 6 pm and shortly before sunset, during twilight (these may be conflicting, since the time of sunset (and twilight) vary a lot depending on season and latitude, as well as the choice of time zone)..
Before a period of transition from the 12th to 14th centuries, "noon" instead referred to what we now call 3 pm. Possible explanations include shifting times for prayers and midday meals, along which one concept of "noon" was defined—and so afternoon would have referred to a narrower timeframe.
The word "afternoon", which derives from "after" and "noon", has been attested from about the year 1300; Middle English contained both "afternoon" and the synonym "aftermete". The standard phrasing was "at afternoon" in the 15th and 16th centuries, but has shifted to "in the afternoon" since then. 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 between 12:00 midday and 12:00 midnight.