In my study for our Philosophy exam, I came across this important, yet humourous definition of the philosophical term from The Shorter Routledge Encylopedia of Philosophy:

“Vagueness: It seems obvious that there are vague ways of speaking and vague ways of thinking- saying that the weather is hot, for example.  Common sense also has it that there is vagueness in the external world.  Intuitively, clouds, for example, do not have sharp spatiotemporal boundaires.  But the thesis that vagueness is real has spawned a number of deeply perplexing paradoxes and problems. There is no general agreement among philosophers about how to understand vagueness.”

Well, that really cleared up my vague understanding of the term!