The process by which cells that were once mature and differentiated become undifferentiated and immature in appearance and behavior is known as

Anaplasia

Explanation:

Anaplasia refers to the loss of cell differentiation, which occurs when cells that were previously mature and differentiated become undifferentiated and immature in appearance and behavior. Anaplasia is a fundamental point for the emergence of malignant diseases. Anaplasia is characterized by:

Cellular Pleomorphism: variation of shape and size of cells and their nuclei. Hyperchromasia: very dark colored nuclei. Abundant Mitoses: Indicates Proliferative Activity Polarity Loss: Formation of tumor masses that grow in an unorganized manner.  Giant Tumor Cells: They have only one huge single polymorphic nucleus, with two or more nuclei. (Not to be confused with foreign body giant cells or inflammatory Langhans cells)