Mental Fitness is ability of the mind to stay neutral when faced with stressful situations.

Mind can be thought of as a simple pendulum, with the relaxed middle position as the neutral state. Any deviation from this relaxed state caused by stressful thoughts and emotions leads the mind to move from side to side. The higher the stress, the higher the deviation.

However, if in presence of a stressor, the mind stays neutral and the situation does not evoke many thoughts and emotions, we can say that we are mentally fit.