The term that refers to the shape of a bacterium is morphology. Bacterial morphology is an important characteristic used in the identification and classification of bacteria. The shape of a bacterium can vary considerably among different species and can also be influenced by environmental conditions. Some common bacterial morphologies include:

1. Coccus (singular) or Cocci (plural): Spherical or round-shaped bacteria.

2. Bacillus (singular) or Bacilli (plural): Rod-shaped bacteria.

3. Vibrio (singular) or Vibrios (plural): Curved or comma-shaped bacteria.

4. Spirillum (singular) or Spirilla (plural): Spiral-shaped bacteria.

5. Filamentous: Elongated, thread-like bacteria.

6. Pleomorphic: Bacteria that can change their shape, often appearing in multiple forms.

Each of these morphological groups can be further divided into subtypes based on specific features and variations in shape. By studying bacterial morphology, scientists can gain insights into the bacterium's function, its environment, and its possible role in causing diseases.