Here's why:

* Phagocytosis is a process where cells, typically immune cells like macrophages and neutrophils, engulf and digest foreign particles like bacteria, viruses, and cellular debris.

* In the scenario you described, the defense cell (likely a macrophage or neutrophil) uses its cell membrane to enclose the bacterium, forming a phagosome.

* This phagosome then fuses with a lysosome, which contains enzymes that break down the bacterium.

Therefore, the process of a defense cell engulfing a bacterium is a clear example of phagocytosis.