the line which divides the earth in two equal parts is called?

I am going to guess the answer the teacher wants is the equator. 

Only one line of latitude divides the earth into equal parts---"top and bottom"or North and South, and it does have a name: "the equator.).

But the prime meridian is only one of the lines of longitude that divide the earth into two equal parts---the "left" and the "right" or East and West.

Here's the definition of meridian---a circle of constant longitude passing through a given place on the earth's surface and the terrestrial poles.

You might want to bring this question up to your teacher and have her explain it a little more.