Besides referencing its titular heroine, the title of Meridian helps us gain a better understanding of our main character's actions.
After all, the first chapter is preceded by a dictionary-style definition of the word "meridian." According to the book, a meridian is a "the highest apparent point reached by a heavenly body in its course." For example, the sun is at its meridian at noon. Meridian is also defined as "the highest point of power, prosperity" or "the middle period of one's life, regarded as the highest point of health, vigor."
Now, that's all well and good, but what exactly does it mean?
Well, Meridian is a meridian. Meridian represents the "highest point" achieved by the activists of the Civil Rights Movement—while her peers quit long ago to pursue their own desires, Meridian alone has dedicated her life to doing good. Similarly, the definition of meridian is connected to the idea of being in the middle, just as Meridian is in terms of ideology. In fact, Meridian is only able to do so much good because she's able sit comfortably between the different ideologies within the Movement.
Basically, we're just trying to say that Meridian is prime. Oh man—we crack ourselves up.