But a black hole is more than just a gravity zone—more than just empty space inside an event horizon. A black hole also includes the source of its tremendous gravitation: a massive amount of densely packed matter sitting at its center. The matter is so densely packed that it forms a single point, called a singularity.

—A Black Hole Is NOT a Hole,
Carolyn Cinami DeCristofano

Using context clues from the passage, explain what the technical term singularity means.