Inhuman means lacking basic human kindness, compassion, or mercy. When we call something inhuman, we're saying it's so cruel or heartless that it doesn't match how humans should treat each other.

The word appears most often when describing extreme cruelty or terrible conditions. History books describe the inhuman treatment of prisoners in concentration camps or the inhuman conditions aboard slave ships. A character in a story might show inhuman cruelty by harming others without any feeling or regret.

The word can also describe something that seems beyond normal human ability or nature. A marathon runner maintaining an impossible pace might be described as having inhuman endurance. A mathematical genius solving problems instantly might seem to have inhuman calculating powers. In these cases, the word suggests abilities that feel almost superhuman.

Notice that inhuman is stronger than just “mean” or “unkind.” It suggests behavior so wrong that it violates our basic sense of how people should act toward each other. When someone acts with inhumanity (the noun form), they've crossed a line between decent treatment and genuine cruelty.