By Julie Kliegman Broadcasters, pundits and writers alike bring up the term with metronomic regularity. But what does it actually mean? Strength comes in many forms and has many sources. But what does it mean to be strong? We have a few ideas. Check back throughout the week for more. “Mental toughness” in sports is so overworked a concept that it feels like it’s been around forever. But it was only in the 1980s that performance psychologist Jim Loehr popularized the term. In the relatively short period since, what we talk about when wetalk about athletes’ mental strength has been garbled into …