Here's a breakdown:

* Oocyte: This is the general term for an immature egg cell, regardless of its stage of development.

* Primary oocyte: This is an oocyte that has not yet completed its first meiotic division.

* Secondary oocyte: This is an oocyte that has completed its first meiotic division but has not yet completed its second meiotic division.

Once the second meiotic division is complete, the oocyte is called an ovum, which is the mature egg cell.