Geometric probability refers to a sub-topic in probability where geometric imagery is used, whether due to the nature of the question or the nature of the answer. A famous geometric probability is Buffon's needle. In this problem mathematician Compte de Buffon asks: if you have a needle of length L and drop it onto a floor lined with parallel cracks all distance D apart, what is the probability that, when dropped at random, the needle will be touching one of the cracks?
Another example of a geometric probability question is: select three points at random on a circle. What is the distribution of the areas of triangles that are created by three points selected in this way? The list goes on, but as long as geometry is involved to solve such questions of probability, then it is considered a part of the sub-topic geometric probability.