There are now over 5 million reference articles on Wikipedia and reproduced below is what Wikipedia, says about defining mental toughness.
MENTAL TOUGHNESS IS A COLLECTION OF ATTRIBUTES
Mental Toughness is a collection of attributes that allow a person to persevere through difficult circumstances (such as difficult training or difficult competitive situations in games) and emerge without losing confidence. In recent decades, the term has been commonly used by coaches, sport psychologists, sport commentators, and business leaders.
MANY PEOPLE USE THE TERM MENTAL TOUGHNESS LIBERALLY TO REFER TO ANY SET OF POSITIVE ATTRIBUTES
Mental toughness is a controversial term, in that many people use the term liberally to refer to any set of positive attributes that helps a person to cope with difficult situations. Coaches and sport commentators freely use the term mental toughness to describe the mental state of athletes who persevere through difficult sport circumstances to succeed. For example, it is often simply applied as a default explanation for any victory, which is highly problematic as an attribution.
THREE RESEARCH TEAMS THIS CENTURY HAVE PRODUCED BOTH A DEFINITION AND A CONSTRUCT DEFINITION FOR MENTAL TOUGHNESS
Published by Paul Lyons