"Erev" means "evening." I am sure there is some technical measure of when evening ends and night begins, but we know that there are two distinct periods of time after the sun sets. Generally speaking, we are still awake for the former (and hence greet each other with "good evening") and we are sleeping or preparing for sleep for the latter ("good night").
In a sort of simple binary view of daily life, daytime is for work and activity, nighttime is for rest and sleep. But there is some time between sunset and bedtime when we are active – including the time we recite our evening prayers. It is a mixture of the experiences of day and the environment of night. And it is that very notion of mixture that is at the core of "erev."
In fact, mixing is the basic meaning of the root. The other usage of this root most familiar in religious context is "eiruv." Nowadays we understand an eiruv to be a boundary, but its origin is in the plate of food placed in a courtyard to enable the residents of various living units to carry to and from each other's residences on Shabbat. Every household contributed something to the plate, thus "mixing" the households together. Similarly, when Shabbat immediately follows a holiday, we make "eiruv tavshilin" by setting aside a piece of cooked food for Shabbat before the holiday so that we can complete the preparations for Shabbat on the holiday. This "cooking mixture" blends Shabbat with Yom Tov.
Noon and midnight are stark and definitive. Evening is vague and often unsettling. It is part yesterday and part tomorrow, making it indefinite in relationship to today. That is the way Jews enter each new day – carrying some of before and reaching for some of after, uncertain about what is ahead. Whether the day just ended was good or bad, the day ahead holds the promise of different. It's a mix. And that's why our prayers are called "Ma'ariv" or "Arvit," and why we praise God for the dependability of the clock and ask God for reassurance of life.