What does title of "Bishop" mean in some African-American churches? I'm familiar with what "bishop" typically means in mainline,  churches. It specifically points to an overseer of several other pastors and/or their congregations. 

Some African-American Christian religious figures, or leaders of "historically black churches", use the title "Bishop", but in my estimation, they may be leaders of a congregation, but not overseers of other congregations.  

Q: What does the term mean in these contexts, and how is the term derived?
Well, as you point out, the term Bishop—which is ultimately derived from the Greek episkopos > biscop [Old English] > bishop—literally just means "overseer." While it came to mean a leader over several parishes in many traditions, its more general meaning could just as easily be applied to someone who oversees a single congregation, which these historically black  churches seem to have done. I don't think there's much more to it than that.