The first online definition I found was "courage and resolve, strength of character". Merrian Webster said "mental toughness and courage" while the OED also says :courage and resolve, strength of character". Some of the synonyms are interesting too, bravery, mettle, pluck, backbone. All the time I'm reading this I'm trying to visualize the first/second grade students in the classrooms I visit.
But before I dismissed the idea as just too non-academic or unrealistic in terms of an expectation in the school classroom I discovered "How Children Succeed" by Paul Tough. Little bells also started ringing in my head because the more I read the more it seemed like there was a direct connection between grit and the growth mindset described by Carol Dweck in Mindset, a book I am currently reading. Further exploration confirmed this connection when I came across this TED talk by Angela Lee Duckworth in which she also makes the connection between grit and growth mindset. Interestingly, she gave no indication of how grit should be developed, and I really do like the idea of the growth mindset.
The original question was posed to me in the context of the word grit appearing in the Common Core Math Standards math practice standards. I've reread the practice standards several times and cannot find any reference to grit. There is certainly a reference to perseverance but is this the same as grit? Does perseverance really involve courage and bravery or is it the disposition of stick-to-it-ness that we all admire in people who have it? Do we really want children to be brave and courageous when they are engaged in math problem solving? For now I prefer 'perseverance' but I will reserve final judgement until after I have read Paul Tough's book.