Immature ovum

An immature ovum is a cell that goes through the process of oogenesis to become an ovum. It can be either an oogonium, oocyte and an ootid. An oocyte, in turn, can be either primary or secondary, depending on how far it has come in its process of meiosis.

Oogonium
Oogonia are the cells that turn into primary oocytes in oogenesis. They are diploid, i.e. containing both pairs of homologous chromosomes. There are 23 chromosome pairs. Thus there are 46 chromosomes. Each chromosome, however, hasn't yet duplicated itself. Thus, there are only one chromatid on each chromosome, making the total quantity of chromatids 46. This is twice the number of chromosome pairs (2N).

Timeline
Oogonia are created in early embryonic life. All have turned into oogonia at late fetal age.

Primary oocyte
The primary oocyte is defined by its process of ootidogenesis, which is meiosis. It has duplicated its DNA, so that each chromosome has two chomatids, i.e. 92 chromatids all in all (4N).

When meiosis I is completed, one secondary oocyte and one polar body is created.

Timeline
Primary oocytes have been created in late fetal life. This is the stage where immature ova spend most of their lifetime, more specifically in prophase I of meiosis. The halt is called dictyate. Most degenerate by atresia, but a few go through ovulation, and that's the trigger to the next step. Thus, an immature ovum can spend up to ~55 years as a primary oocyte (the last ovulation before menopause).

Secondary oocyte
The secondary oocyte is the cell that is formed by meiosis I in oogenesis. Thus, it has only one of each pair of homologous chromosomes. In other words, it is haploid. However, each chromosome still has two chromatids, making a total of 46 chromatids (2N). The secondary oocyte continues the second stage of meiosis (meiosis II), and the daughter cells are one ootid and one polar body.

Timeline
Secondary oocytes are the immature ovum shortly after ovulation, to fertilization, where it turns into an ootid. Thus, the time as a secondary oocyte is measured in days.

Size
The secondary oocyte is the largest cell in the body, and in humans is just visible to the naked eye.

Ootid
An ootid is the haploid result of ootidogenesis. In oogenesis, it deosn't really have any significance in itself, since it is very similar to the ovum. However, it fills the purpose as the female counterpart of the male spermatid in spermatogenesis.

Each chromosome is split between the two ootids, leaving only one chromatid per chromosome. Thus, there are 23 chromatids in total (1N).

Timeline
In other words, the ootid is the immature ovum from shortly after fertilization, but before complete maturation into an ovum. Thus, the time spent as an ootid is measured in minutes.

Ovum
The ootid matures into an ovum.