Thankfulness is the feeling of being glad and grateful that something happened — especially when you can see how much worse things could have been. That last part is what gives the word its particular flavour. If you pass an exam you feel pleased; if you nearly crashed the car and did not, you feel thankfulness. There is relief inside it. It is the noun of *thankful*: he is thankful, and what he feels is thankfulness. Two things to notice about how it is really used. First, it is a quieter, more inward word than *gratitude*, and often it is not aimed at any particular person — you can feel thankfulness about good weather, or about being alive, with nobody to thank. *Gratitude* usually points at someone who did something for you. Second, it sounds slightly old-fashioned and slightly religious in modern English; you meet it in church talk, in speeches, in writing about family, more than in daily chat. In ordinary conversation people are more likely to say "I'm so thankful" or "I'm grateful" than to use the long noun. It is an uncountable word, so it takes no *a* and no plural: not "a thankfulness," but "a feeling of thankfulness," "deep thankfulness."