A velleity is a wish you have — a wish that you aren't working to make come true. If you have a velleity to run a marathon, the marathon sounds like a good idea — but you probably aren't going to start training.
The word velleity comes from the Latin word velle, to wish or to will. A mild impulse to do something, unaccompanied by any real motivation to follow through, can be called a velleity. If you dream of going on safari in Africa, but you never even look into how much it costs, that dream was just a velleity. Your dream of speaking fluent Spanish is a velleity if you never get around to opening your textbook.