I have found most dictionary definitions of joy to be sadly lacking:
The passion or emotion excited by the acquisition or expectation of good; pleasurable feelings or emotions caused by success, good fortune, and the like, or by a rational prospect of possessing what we love or desire; gladness; exhilaration of spirits; delight. http://www.brainyquote.com/words/jo/joy181774.html
The feeling of happiness, extreme cheerfulness.
My conception of joy is of something more profound.
Joy is to be found in a place of inner quiet, the point of light within – the divine spark? It is more the manifestation of a quality of spirit than an emotion, existing only in the ‘now’.
My sense is that we all possess a capacity for joy, carrying it with us constantly. But we have to nurture and, perhaps, exercise this capacity if we are to cultivate joy in our lives. We can do this by giving our attention to what we do and to the world around us (mindfulness). The more we tap into joy, the more able we become to do so. We also need to create in our lives a space into which joy may enter. This is the point of connection between joy and meditation or stillness.
Whilst happiness is certainly linked to joy, it is possible both to be happy without any experience of joy and to experience joy even from the midst of a situation of great sadness and pain. Indeed, in those darker moments, joy can be our buffer against madness, for pain cannot feed on joy.