Psychological capital (or "PsyCap") is what most people call "mental toughness". 

It refers to four (4) characteristics people need in challenging situations. They are - 

1. Hope
2. Efficacy (Confidence) 
3. Resilience 
4. Optimism 

NOTE: Use the acronym "H.E.R.O." 

Research has shown that people with high levels of PsyCap perform better and are more satisfied with both their job and life in general. 

(F. Luthans, et al, 2007)