If you want to determine how toxic a certain chemical is, you develop a curve like the one shown above. This is called a Dose-Response Curve.
Notice the label on the y-axis: "percent of animals responding." This curve shows what percentage of an animal population "responds" to a toxicant as it is administered at higher and higher dosages. Below the "No Observable Effect Level" labeled "NOEL", animals do not respond to the dosage. At a high enough dose, every animal "responds."
How is animal "response" determined? Well, if after being administered the suspected toxicant, if the animal is alive, it has not responded. If it is dead, it has responded.
Now THAT is what I call a euphemism.