What is meant by the term 'scarcity' in economics?

Quick Summary
The term scarcity in economics is used to describe the problem of having unlimited human wants or desires and a limited supply of resources or commodities. Scarcity explores the idea that human wants may be satisfied for a time, but new wants will arise, and we will never be able to produce enough goods and services to satisfy all human desires. 
Original question: “What is the economic concept of scarcity?”

Scarcity is essentially the notion that resources are available in limited supply. All resources are scarce, though it depends on the context from which you view them, as they may be scarce in some contexts, but not in others.*
If resources weren’t scarce, we could create an unlimited amount of anything we knew how to make (and many things we don’t even try to make, for lack of resources.) But that’s not the world we live in.

Meanwhile, economics, at its core, is the study of how best to use those limited resources. Frequently, economists try to optimize relatively opaque notions of ‘utility’ or ‘wellbeing,’ given that the inputs required to produce these notions are limited. In super basic terms, economics is an academic attempt to put the world’s limited resources to their best use. Put differently, economics is the attempt to make the most of the scarce resources at our disposal.
All economic problems stem from how much of a given resource exists, and how they are spread out around the world/universe. (In the near future, we will likely mine resources from asteroids and objects from beyond our planet, so we will end up reevaluating what is and isn’t scarce.)

*An example of this context is that there is plenty of water in the universe, but we can only really access the water on the planet earth. Time is scarce, in that, as of now, humans have limited life expectancies, but it’s not scarce in that grand tasks like the building of the great wall, could be tackled by many workers.