I think it means the level (depth) to which the platelet count falls.

Nadir Affects Blood Cell and Platelet Counts
When chemotherapy is given it not only affects the rapidly dividing cancer cells but it also affects some of the normal cells of the body. These effects particularly occur on normal cells that divide rapidly such as, the hair, the lining of the mouth, the cells lining the intestinal tract and the blood cells (white and red blood cells as well as platelets).

In the bone marrow, the spongy inner core of the larger bones in the body is where blood cells are made. There are very immature cells called stem cells, from which the various types of blood cells develop. These stem cells do not reproduce quickly and are less likely to be affected by chemotherapy. As cells are maturing there are certain phases in which they divide faster. It is during these times that the cells are most sensitive to chemotherapy. The more mature cells can continue to become fully mature cells for several days after chemotherapy is given. When these cells live out their life span, the circulating supply is depleted and the blood counts fall to a low point, the nadir.

http://chemocare.com/chemotherapy/what-is-chemotherapy/what-...

Degree of Platelet Count Fall
Degree of platelet count fall, measured from the peak platelet count following heparin initiation to the nadir platelet count, is ≥50% in most patients. Ten percent evince a more modest decline of 30–50%. The nadir platelet count need not meet the traditional laboratory definition of thrombocytopenia (<150,000/µl). For example, surgical patients with postoperative thrombocytosis may subsequently experience a major decline in the platelet count from the postoperative peak that does not fall below this threshold. Indeed, as opposed to other drug-induced thrombocytopenias, the median nadir platelet count in HIT is approximately 60,000/µl and seldom falls below 20,000/µl unless concomitant DIC is present.

https://www.sciencedirect.com/topics/biochemistry-genetics-a...

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Note added at 54 mins (2019-03-20 17:52:33 GMT)
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Depth = the maimum fall/drop/decrease in platelet count.

After many types of chemotherapy, the platelet count generally starts to drop by day 7 and reaches its nadir at day 14, with a gradual return back to baseline by day 28 to 35 (Figure 1).

https://www.cancernetwork.com/oncology-journal/managing-thro...