An inability refers to lacking the capacity to do something. A deaf person has an inability to hear.
If you know that abilities are things people can do — like the ability of a basketball player to dunk the ball — then you might already have figured out that an inability is something a person cannot do. Most people have an inability to dunk a basketball. People who eat too much have an inability to resist food. Often, this word refers to having mental limitations: an inability to think or figure something out. We all have different abilities and inabilities.