What is the microscopic study of cells?

Cytology (the prefix cyto- means "cell") is the study of cells, though the more simplistic term 'cell Biology' is often used too.
Cytology (from Greek κύτος, kytos, "a hollow"[1]; and -λογία, -logia) means "the study of cells".[2] Cytology is that branch of life science, which deals with the study of cells in terms of structure, function and chemistry.
cytology