Soundness means the quality of being in good condition, based on solid reasoning, or free from damage or defect. When a doctor examines you and declares you in sound health, she means your body is working properly without illness or injury. When a carpenter checks the soundness of a wooden beam before building with it, he's making sure it's strong and won't crack or break.

The word often describes the quality of thinking and arguments. A sound argument uses good logic and reliable facts to reach its conclusion. If your friend claims that eating ice cream for breakfast is healthy because it contains milk, you might question the soundness of that reasoning. Scientists test the soundness of their theories through careful experiments.

Buildings, bridges, and other structures also need soundness. Engineers inspect the soundness of a bridge's foundation to ensure it can safely carry traffic. A house built with quality materials and good construction has structural soundness that can last for decades.

The word carries the sense that something works as it should, whether that's a logical argument, a physical structure, or a person's judgment. When something lacks soundness, it's unreliable, flawed, or likely to fail when put to the test.