The term nurse cell is used in several unrelated ways in different scientific fields:
Nurse cells are specialized macrophages residing in the bone marrow that assist in the development of red blood cells. They absorb the nuclei of immature red blood cells and may provide growth factors to help the red blood cells mature. In the bone marrow, immature red blood cells (erythroblasts) can be seen grouped in a cluster around a nurse cell.
The epithelial cell found in the cortex of the Thymus is also called a "nurse cell." These cells produce Thymic hormones that cause T Lymphocytes to mature and differentiate.
In microbiology, a nurse cell is an infected cell in the disease trichinosis discovered by Dickson Despommier. A trichinella larva enters a cell and develops there, probably as a way of concealing itself from the immune system. The parasite has evolved a way of stimulating blood vessel development around the cell, in order to receive the nutrients it needs. In trichinosis, nurse cells are invariably skeletal muscle cells; these are the only type of cell that can support the parasite.
In respect to invertebrates, nurse cells are polytenic germline cells that contribute to the development of the oocyte, producing multiple nuclei. In fruit flies (Drosophila), nurse cells surround the developing oocyte and synthesize proteins and RNAs that are to be deposited in it.  They contain about 250-500 nuclei, all 15 of them in all oocytes creating 5,000 nuclei in total. They dump their cytoplasm containing the nuclei into the oocyte via ring channels.